It seems to me you ought to be railing against English grammar in general rather than criticizing me. I do not make the rules, I just use them the best way I can (and that doesn't mean I always use them correctly).

I don't claim to be an authority on English grammar, nor am I as pedantic about its use as you imagine. My background is technical, and my English grades at school were nothing special (I was never asked to write for the school's magazine). Whilst it may seem to you that I am being overly donnish about usage, I have learned the hard way that using sloppy English gets noticed and doesn't win one any favors, in fact, I've found that not being careful about one's usage can be outright disadvantageous. People who do have a good command of English grammar may not mention bad instances out of politeness, but they never let them go unnoticed. Grammar is so ingrained in their psyche that bad usage automatically waves red flags.

That said, I've been lucky enough to have encountered people who have had no compunction about drawing my attention to my grammatical errors. Many instances come to mind and several were embarrassing. Decades ago when working in Europe, I had colleagues—native German speakers—proofread my technical documents. The trouble was that besides a few errors in the subject they also drew my attention to my grammatical errors and inappropriate use of colloquialisms. It's a sobering moment when one meets non-native English speakers who speak English better than one does. That perhaps is an unusual case, but I've had similar experiences in my English-speaking environment.

If you think such matters unimportant then so be it. I would contend however that if one wants to be taken seriously then paying attention to one's grammar is important. Incidentally, I'd suggest that one never sees instances of the grammatical errors we've been talking about actually appear in books and publications from big well-known publishers, as book editors simply never let them go through to printing.

Your reference to the apostrophe to indicate a plural is nevertheless deemed grammatically incorrect by most references. That it's earned probably the most derogatory nickname of all instances of bad usage—the greengrocer's apostrophe, or apostrofly —seems to attest to how widely it is detested. Whilst some cases are tolerated to avoid confusion, there are usually ways to avoid using them. For example, Mind your p's and q's is easily resolved with Mind your Ps and Qs, the same for DVDs. It's not the unusual cases that are the issue, it's that by far most instances of bad usage are committed by people who simply don't know how to use the apostrophe because they were never taught its correct usage at school.

It's misfortune that English is lumbered with the apostrophe but its use is simple to understand and its rules can be taught in just one lesson, unfortunately, it's often taught in such a bungled and convoluted way that it's little wonder students are often left in confusion. Such bad teaching ought to be held up as an exemplar of how not to teach English. I vividly recall learning the possessive apostrophe in primary school in just one lesson, and I've never forgotten it. The teacher said 'Just ask yourself a question who owns the object and the answer will automatically tell you where to place the apostrophe—that is, it's located immediately at the end of one's answer'. That's it, end of story: 'who owns the bat?', answer 'one boy' thus the boy's bat; if two boys have ownership then it's the boys' bat. It's remarkable that so many manage to screw this rule up.

Some grammarians suggest eliminating the apostrophe altogether. That's one solution, the logic being that one understands what's meant from its context and that's often true but not always so, same applies for the fewer/less case, but again ambiguity can often creep in.

Given English is, in parts, already disastrously messy and ambiguous, I'd postulate it makes little sense to add even more confusion to the language by intentionally making it even more so. After all, the purpose of language is communication, and ideally that ought to be as unambiguous as is possible. As mentioned, the possessive apostrophe can't get much simpler and still be a rule, if anything it can even be improved, in doubtful instances where 'possession' could apply to either a single or multiple owners then ambiguity could be stated specifically by deliberately omitting the apostrophe altogether.

"now that's getting really pedantic… …I'm not sure use of "that" in this context has ever been considered bad usage outside of highly prescriptive "Good English" guides."

Your comment about making a distinction between that and who as being pedantic is informative because their meanings have always been well defined and that still hasn't changed. If many are now beginning to perceive the distinction as being unimportant then this is a recent phenomenon, and I'd reason is likely generational. Any reasonably educated person of several generations ago would consider a misusage of these words as crass and unacceptable on grounds that a language should always be able to distinguish between humans (as living animate beings) and those of inanimate objects if for no other reason other than out of consideration for the dignity of human beings/living things.

The most recent and authoritative grammar references make this clear distinction and it's still the accepted practice. When referring to both people (in its collective sense) and to individuals then who is always used. It is however acceptable to refer to a nondescript group using that. Confusion often seems to arise when making reference to, say, a company where no clear distinction is made between the inanimate entity and those humans who are employed by the organization. For example, 'the policy of Microsoft [corp] is that…' is typically confused or confabulated with, say, those at Microsoft who determine policy'.

Incidentally, I use The Cambridge Grammar of the English Language as my grammar reference, I purchased my copy of this 1800-plus page tome not long after it was published about two decades ago. It's the most authoritative reference of English grammar ever written, it's also still the most current one: https://www.cambridge.org/features/linguistics/cgel/reviews...., https://en.wikipedia.org/wiki/The_Cambridge_Grammar_of_the_E...

The question is how far can a language decline or fragment to accommodate the lowest common denominator before becoming ineffective and falling into disuse or metamorphosing into some other altogether different language. I cannot answer that, but little doubt exists change happening and exceedingly fast at that. I'm in full agreement with you when you said "The countable/uncountable distinction in English is eroding and may well be dead in a generation.", it's only but one instance of many such changes. Whilst change has always been a characteristic aspect of English—more so than many other languages—and that sloppy usage has always been been a contributory factor, that change picked up with great apace some 50 years ago. That's when the rot set in big-time—when educators thought it a smart idea to simplify the learning of English by essentially eliminating the teaching of grammar altogether. As we've now seen the results have been nothing short of disastrous.

If we also consider the multitude of other factors† at work on English then we've a recipe for rapid changes to occur within the language. For instance, it's unfortunate that English doesn't make use of diacriticals, then there's the propensity of English to steal words and phrases from other languages at the drop of a hat and then integrate them without due attention to context and meaning and or without attention to pronunciation, and finally there's diversification brought about through its adoption by different ethnic groups, etc. My personal views is that English is changing and diversifying so rapidly that in several hundred years it'll not only be unrecognizable to today's users but also it'll have fallen from prominence and become a backwater language of little significance.

__

† Factors such as those mentioned make English such a dog of a language, especially so for those who must learn it as a second language. Like it or not, English has never had the 'discipline' of say French with Cardinal Richelieu's four-hundred-year-old watchdog the Académie Française hanging over the language, nor has it had the stability of German where a German speaker is still able to read thousand-year-old German texts without much difficulty. And without diacriticals, English ends up having dozens of common words that are confusing and or are difficult to spell or pronounce such as 'through' and 'thorough' and the proper noun 'Warwick'.

Then there are those words imported into English—imported not through any specific need as perfectly adequate ones already exists in English—but solely because they are fashionable. 'Tsunami' is a quintessential example of such a fashionable import, it's not only displaced perfectly adequate English words but despite its translators having provided a good translation into the Latin alphabet, English users choose to ignore the translation and make no attempt to pronounce the word correctly. Instead of pronouncing the hissing snake-like sound of 'tsu' what usually emerges is a bastardized slurred-out 'sooo'-like sound with little or no deference to culture from which it came. It's little wonder that much of the world cannot but help notice the palpable and very distasteful arrogance of native English speakers.