>> because a bilabial stop doesn't use the tongue and so [n] is easily coarticulated

> That doesn't sound quite right—this assimilation surely wouldn't be nearly as globally prevalent as it actually is if that were true.

> Try it.

You know, I mentioned a specific theory here that you've completely ignored. The coarticulation is easy. But it is difficult for a listener to tell the difference between coarticulated [nb] and [mb]. If you're willing to let multiple generations pass, this means that /nb/ will become /mb/ regardless of how easy it is to pronounce.

You will also note that this theory of what's happening mostly cannot be disproved by recordings, which you appear to want to do. You'd want an X-ray or MRI study, something which shows you what the tongue is doing.

> I think you'll find it quite challenging to coarticulate [n] with [b]—tongue positioning at lower teeth is pretty obligatory

This is just obviously false. You have no problems producing [b] with your tongue positioned however you like. You can position it for [t], you can position it for [tʃ], you can position it for [k]. And of the three coarticulations I just mentioned, all of them are well attested, though only the middle one is attested in English ("pshaw", a scoffing sound).

> and much easier to sequence them

This is worthy of comment; there is a linguistic concept called "coarticulation", but all cases of coarticulated consonants seem to have a conventional sequence associated with them. I have no real knowledge or opinion on how real the conventional sequencing is, or how much sequencing is allowed before you stop calling the sounds coarticulated. I suspect that indeed it is easier to sequence two events than to coordinate them to occur at exactly the same time; this is true for all types of events, not just language-related ones. I don't think that the linguistic concept requires absolute synchronization of particular points in time; my understanding is that producing any given phoneme requires some motion and therefore takes place over a nonzero span of time, and "coarticulated" consonants are those for which the durations overlap, not necessarily those for which the durations perfectly coincide.

But I will note that while sequencing of /nb/ is obviously necessary in a way that is not true for /pt/, since /n/ must have nasal airflow and /b/ must not, there is no reason for "coarticulation" of /nb/ to be more difficult than it is in the attested coarticulation /tm/ (exactly the as /np/ for our purposes; /tm/ also features a voicing difference between /t/ and /m/).

> Consider "inpainting", "unbiased", and (as suggested earlier) "government", each of which is a synchronically transparent /n/ across a morpheme boundary

I don't think "government" is a valid example, and you should stop trying to lean on it. In my view, the pronunciation of "government" has as much to do with the morphemes suggested by its spelling as the pronunciation of "comfortable" does with the morphemes suggested by its spelling.

I have no problem with "unbiased"; that's a great example of what we're talking about.

> Clearly you can see the unnaturalness of lack of assimilation to call the attempt to do so "special effort"!

I don't agree with this. I claim that it is common for Anglophones pronouncing "unbiased" to make contact between the tip of their tongue and their alveolar ridge while they pass over the /n/ in the word. (And here, we're on firm ground saying that the internal phoneme is /n/ and not /m/, since it's part of a productive prefix un-.) I further believe that they make no special effort to do so. They may or may not allow a longer duration of nasal murmur than they do in other contexts, to make the /n/ clear; doing this would constitute a special effort. I believe that some speakers will do this and some won't bother. Of those who do, only a small amount of effort will be given to the task.

But the case of English speakers attempting to pronounce Japanese is different. They will go to great lengths to demonstrate that they want to comply with the bizarre textual representation they see. They are happy to produce highly unnatural speech in order to do so. (Which isn't really a problem; they don't really have an alternative to producing unnatural-sounding speech in early attempts to pronounce a foreign language. But this is something they shouldn't encounter problems with.)

> And in a wider context, the principal users of romaji Japanese aren't anglophones; they're Japanese-speakers who for some or other reason need need to coerce Japanese text into an ~ASCII-subset representation, targeting primarily computer systems with that sort of limitation (most common case being keyboards via IME, hold that thought)

> (This is also why pinyin can be the way it is, yet their IMEs have routinely have modes to ignore s-sh/n-ng/n-l distinctions.)

This isn't a flattering comparison for the all-n Japanese transcription system. The pinyin for 吕 is lü. Chinese people don't use German keyboards, which makes the pinyin impossible to type. So where ü contrasts with u, pinyin input methods require you to input V. And Chinese people have responded to this by adopting v-based spellings; it is common to see pseudo-pinyin like "lv" where that pinyin has been generated by an ordinary Chinese person for their own purposes, such as a sign over their business or an online username.

But the letter V is formally not a part of pinyin at all, which means that text generated by the government never uses it and neither do instructional texts.

It is true that this situation is the reverse of the one we're discussing - the Chinese are making a distinction that is required by their language but forbidden by their keyboard, and the fact that they are aware of the distinction makes it easy for them to know what to do. The Japanese are failing to make a distinction that doesn't exist in their language but does exist on their keyboard; this is precisely parallel to the pinyin IME settings you note that will allow the user to ignore phonemic distinctions that they don't make. Again we see that the system maintains the distinction and it's the job of the input method to interpret what the user wants to say.

Chinese IMEs also offer a "double pinyin" input method, in which you type one letter to indicate the onset of a syllable and a second letter to indicate the rime. All syllables are two input-letters long; this model matches the traditional Chinese view of their own phonology. You could just as easily base your system of English transcription on this: instead of "Xi Jinping", 习近平's name would be "Xi Jnp;". Instead of "Sun Yat-sen", we'd talk about "Sp Yixm".

That's what it looks like when you base spelling on what it's convenient for foreigners to type as an intermediate input to their own, different spelling. (As is the case with Japanese input methods.) There are zero people who believe it's a good idea. It's not a better idea in the Japanese case.