User-agent is super useful to human people. But corporate people don't have a use for it. They will get that information via running arbitrary code on your insecure browser anyway. So, because mega-corps now define the web (instead of the w3c) this is life.
But it doesn't have to be. We don't have to follow Google/Apple-Web standards. Anyone that makes and runs websites has a choice. And every person can simply choice not to run unethical browsers.
For what? Honest question. You have to be like a 5th-level user agent wizard to make any sense of user agent strings, since every browser now names every other browser. How do you do anything useful with this in a way that's forward-compatible?
Not sure why you are being downvoted since your statements are correct.
Few advertisers rely on user agent for ad targeting since it can be easily mocked with each HTTP request. It is used for fingerprinting, sure, but from my experience, mostly as a way to identify bot traffic.
It is also true that the advertisers that fingerprint people rely on JS that executes WebGL code in order to get data from the machine.
Finally, you are right that it doesn't make sense that a company like Google dictates these standards since they have a conflict of interests worth almost a trillion dollars.
Unfortunately they are either unethical or have other problems (or most commonly, both); I have made suggestions how to make a better one. See other comment elsewhere they explain
But it doesn't have to be. We don't have to follow Google/Apple-Web standards. Anyone that makes and runs websites has a choice. And every person can simply choice not to run unethical browsers.