I go back and forth. I do web development, and I do occasionally rely on the user agent, I completely agree that there are some situations where there isn't really an alternative we can use. It's not necessarily just a problem of specification, sometimes there are browser-version specific bugs that just have to be accommodated, and there is no way to test for those bugs and there is no way to get a signal for those bugs because they're not specified behavior.

However, I also browse the web on Linux, and there are also situations where sites just kind of decide that they do or don't support me based on whether or not I'm in an allow-list that was lazily cobbled together and that hasn't been updated in over a year. This seems to me to be the same category of problem; it should be better, sites should know not to do that, but they don't. And sure, I can solve the problem specifically for myself by going though an annoying process to lie about my user-header, but it's a big damper on all of the other "users" (ie, random family members/friends) that I support who aren't able to do that. And it defeats the primary purpose of the user-agent if I'm lying to sites about it because they block off capabilities for no good reason from agents that they don't recognize. If I'm constantly lying about my browser as the "solution" to that problem, I'm already kind of breaking user-agents in the exact way you're worried about.

And there are also obviously the privacy problems that come along with user-agents, which I'm not going to get into, but they're substantial and there is no way to solve them without making user-agents much less useful for website operators.

So I don't know what the solution is, or even if there is a better solution available than what we currently have, but there are real downsides to the current setup. I think it's silly to pretend that browser vendors won't ever have quirks that make user-agents necessary. But I also think it's equally silly to pretend that website operators will ever use them responsibly, and equally silly to pretend that people won't get locked out of sites for no reason because of them.

There is definitely some naivete in getting rid of user-agents, but there's also some naivete in keeping them, and the arguments for "well, they're still necessary" sometimes ignore just how much wasted effort and hacky crud goes into making the web work with them. At the same time though, there are bugs I've fixed in my day-job that could not be fixed without them. It's just, that doesn't mean the downsides aren't also there and that they don't also matter.

I'm personally interested in how client hints progress. They're not perfect, but they seem to be decent, and for all of my criticism of Chrome's Privacy Sandbox concept, I think this is an area where it makes a lot of sense -- make certain hints possible to check, but have a cost associated. It's still not the exact browser version though, there are still bugs that I wouldn't be able to fix with that system. But there are some bugs that I use user-agents for that this system would work for, and I'm willing to make my job slightly harder if it makes a bunch of other things better. Or at least, I'm willing to see how client hints play out and see how much harder they do or don't make my job.