Let's also assume many people with different needs and desires are reading this thread. Yes, it definitely makes sense for some people to order 16 for the reasons you suggest. And for others, 8 will fit better, even for 4 or 5 years.
Let's be clear - 16gb is not excessive memory by any stretch so it will always 'fit better' over the long term - there's no advantage to less memory except cost. Granted the benefit might be small for many - for light users perhaps not even noticeable.
The only other side of this question though is if people prefer to use that $200 for something else or can't afford it - which is of course entirely reasonable. I'm not debating that for many 8gb will be 'enough' - that's an entirely personal choice - but 16gb does give an extra buffer that's useful in terms of long-term usability.
Personally my advice to anyone on this is that if you want to keep using a computer for five years, and since it can't be expanded later, it's
really worth considering that $40 a year in upfront cost is not a lot.
But again, I agree, it's a personal choice, and if someone is confident they don't need it and have other priorities, that's their call.
My personal experience with non-upgradeable is limited to this one macbook air I'm using now (every other mac I've used has been upgraded two or more times). And my one regret with it is that I didn't (couldn't actually, separate story) get the next step up for both memory and SSD. No complaints, it's served six years and is still perfectly usable for lots of tasks - but man oh man would it have been worth it to pay a bit more upfront, and would have saved me a fair amount of time and money, too, since I've had to do various workarounds.