On the question of how well Proton works vs. native, the thing about some of your points is that people expect Proton not to work. I mean, yeah, they figure it will work most of the time, but failure is part of the normal parameters of using a thing like Proton;

Maybe initially, when it all started out?
But I cannot even remember the last time Proton was not able to run a game. It must've been years ago.
For a while I checked ProtonDB before buying a game - I don't even do that anymore.

Besides, plenty devs officially support Proton at this point - of course people then expect it to work, just like they would with a native port.

At this point in time (with the exception of MP and VR games), Proton is very much a "it just works" thing. And widely perceived as that as well. You are honestly the first person I see viewing it so negatively.
And I talk to lots of Linux folk, and even more to gamers outside the Linux sphere (usually talking about the OS itself), all of which very much seem to think they could run their games very well on Linux (and they expect occasional fiddling), but they are intimidated by the OS itself.

You are really allowing your own exceptional experience to cloud your judgement of what most people see - for that, a look at ProtonDB reports is more than enough. Way over 90% silver+ for reported games puts failure outside of a "normal parameter" with Proton at this point.

Nobody reports it to the devs when they try a Windows game in Proton and it doesn't work; they know it's not supported.

That depends a bit on what your definition of "reporting to devs" is.
You can find plenty of bug reports of people running a game through Proton on Steam forums. Most of which of course not related to Proton.
But in the past, for example at least until Proton got its video support fixed, you could see a lot of issues raised ala "game works fine through Proton, but videos don't play" (remember that "TV test screen" thing?). Usually devs reacted to it, pretty much always others, with a variety of end results.
But it did get reported - official support or not.

If you are specifically talking about some dev-specific bug report channel, then yeah, you are probably right - but it's not like those are ever used by most people to begin with.
The vast majority of users, as in 90+%, do not ever use bug reporting features or "proper" channels (and I can tell you that from decades developing user-facing software) - and I'd wager gamers do it even less.

And again, lots of official Proton (often in the form of Steam Deck) support by now.

Then, it also depends on which dev you are talking about.
There is the Proton bug tracker, the Github one - obviously most people will never have a Github account, but considering that (and likely due to the higher amount of techies among Linux users) the amount of reports there are surprisingly high.
So, one could argue those very much report to the devs - the devs of Proton.
If some Proton-related bug is more likely to be fixed by the game dev or the Proton dev depends on the case.

It's not a symmetrical thing when it comes to how and whether people complain,

Oh, man, people complain.

Doesn't matter if anyone thinks it is "reasonable" or "plausible" - we are long past the times where one could expect people to behave in those categories, especially online.
If you think people wouldn't complain to a developer because something is not officially supported, then you live in extreme contrast to reality.