> sites that want it will get it this way and even if you respond with "no" that's useful to them as well for fingerprinting
To an extent, sure. But to follow the model of third party cookies, let's say client hints are used extensively instead of user agent and all cross-domain iframes are blocked from client hint sniffing. All the third party iframe is going to be able to detect is whether user has a client hint capable browser or not. That's a big difference from the whole user agent they get today.
The idea is that this won't be a Chrome-specific API. It's been submitted to standards bodies, but Chrome is the first to implement. For example, Firefox have said they "look forward to learning from other vendors who implement the "GREASE-like UA Strings" proposal and its effects on site compatibility"[1] so they're not dismissing the idea, they're just saying "you first".
https://mozilla.github.io/standards-positions/#ua-client-hin...