This topic certainly generates some strong opinions
The way I see it:
(1) Macs are marketed as premium machines, and priced as such. Whether they are "worth" the price is highly subjective and depends on all sorts of variables, not just the raw specifications and the cost of similar components on other machines
(2) I think we can agree that the component cost difference between 8GB and 16GB costs Apple a lot less the price for end-users. A large part of the anger here is caused by this apparent price gouging, and while Apple's upgrade prices are high, other manufacturers are not much better (e.g. an upgrade to a Dell Inspiron from 8GB/256GB to 16/512GB is also about $400). Unfortunately, this is just the nature of marketing and price segmentation.
(3) MacOS *does* run quite well in 8GB, together with "a few" applications, but anything more than relatively light use (e.g. a dozen browser tabs, music app, chat/messages, simple documents at a time) will likely cause swap memory usage and *eventual* slowdown. Of course, as applications & web-sites become more RAM hungry, this issue will become more marked, which is another cause of dissatisfaction and concern that the machine could become "weaker" in a couple of years.
(4) Apple could increase the base RAM to 16GB and add a small price increase without losing profit they make on the 8GB machine...but they *would lose* the big profit they make from a chunk of people upgrading to 16GB at $200 a pop.
Ultimately, people will vote with their wallets.
If buyers understand that 8GB isn't much these days, and all of Apple's competitors offer more at a cheaper price, then the "premium allure" of Apple will start to diminish. It will be seen as poor value, despite the other advantages the machine may (or may not) have in buyers' eyes.
If technical buyers understand that a RAM upgrade is essential for their intended usage, they have to add that the price to an already expensive machine, knowing that it is a largely artificial cost imposed by Apple, which causes resentment.
At the end of the day, if the overall opinion is formed that a Mac computer represents poor value for money (taking into account specs, functionality, performance, build quality, reliability, TCO, premium status), their market share will decrease and Apple will either take the loss, or adjust prices either up (to compensate for the loss) or down (to attract more market share).
I liken it to eating at an expensive but highly recommended restaurant. I've eaten in a few places that cost a four-figure sum for two people. At that price, the experience has to be exceptional, and even then it's hard to justify. On a couple of those occasions, I didn't think it was that great for the money. The result? I didn't go back to those restaurants, and looked for better value elsewhere. Other people may (literally) dine out on telling their friends that they ate at that exclusive restaurant that cost a fortune, and the actual food wasn't the important thing to them.
That is the risk that Apple is taking.