I don't understand your opening para. Either way, I don't think defective software has anything to do with it.

That seems where we disagree.

With other kinds of product I buy, at least in my country, there are consumer protection laws about the being of sufficient quality and fit for their intended purpose. If a product doesn't live up to those standards, I am entitled by law to have the situation put right or compensated one way or another, for example through repair, replacement, or ultimately a refund of some or all of the money I paid to buy the product. If a product is so bad that it causes other harm as well then in some cases I would also have grounds for further compensation. (The situation is also somewhat different when selling to businesses vs. private individual in my country, so I'm oversimplifying here.)

With software, developers have historically been given a lot of slack, in part because none of us know how to write bug-free programs so expecting everyone to achieve that standard before being able to sell anything is unhelpful. However, the same basic legal principles do still apply, and they have sometimes been used in practice. Part of the reason that big software developers like Microsoft don't fall foul of those rules more often and wind up paying back a lot more in refunds and/or other compensation is that they do make reasonable efforts to fix defects in their software for a reasonable time after purchase.

My point is that providing a reasonable degree of support is not really optional for them and they are not really being generous in providing it. If they stopped doing so, and their customers then started suffering real damage because of bugs or vulnerabilities in Windows, then Microsoft would risk being sued until either they fixed the problems or their business failed.

This has absolutely nothing to do with any other kind of update. Microsoft has, to my knowledge, no obligation after a customer has already purchased their copy of Windows to provide ongoing development of new features, drivers for other vendors' hardware, or their UI. They might choose to do that, and they might choose to offer those things to customers in return for money, data, or any other agreed form of compensation.

But whether or not they do that, it doesn't change the basic obligation they do have to provide a decent product if they're charging real money for it, and to make reasonable efforts to fix defects or compensate for them if their original product is flawed. The customer is entitled to get what they paid for, not a broken version of what they paid for, and not a version where a defect was fixed but some other unwanted change was also made so it's still not what the customer was originally supposed to be buying.

A quick google....

I'm not sure how much credibility I'd give reviews from that kind of source. Of course they look favourable, because they seem to make little effort to be at all critical of anything and almost completely gloss over the widely reported problems and backward steps in favour of... ooooh, shiny!

For example, several of them highlight robustness as a big point in favour of Windows 10. Given the huge amount of negative comments from people who were in the beta/preview programme and the multiple, widespread, system-breaking forced updates that have already been pushed out within just a few days of launch, I don't see how any unbiased review could possibly conclude that stability is a strong point for Windows 10 so far, and I see no basis for the blind faith several reviewers seem to have that Microsoft will fix the fundamental risk of bad updates bricking boxes. Few of the reviews make a big deal of removed features like Media Center, or hardware incompatibilities with older devices, which are the kinds of issues that won't trouble the majority of users but will be very bad for those who are affected. I don't see much mention there of clunky search tools or the confusing division of what used to be in the control panel into multiple areas. None of the reviews that I looked at even mentioned things like privacy concerns, or spamming ads at you unless you pay subscription money to turn them off, or the WiFi Sense security concerns.

Try Googling a few other relevant terms, like say "Windows 10 laptop reboot loop" or "Windows 10 WiFi Sense security" or "Windows 10 search", and see how overwhelmingly positive the commentary looks then.