“useful” is relative. I prefer a world where websites can know which platforms users are coming from, as it helps them know where to focus their support efforts.
There are billions of users but probably only a few OSes mentioned in UA strings so it seems like a decent trade off to me. My exact UA string is likely shared by millions of users even though my OS is somewhat rare on the world stage. Until the day comes that web browsers work exactly the same way on every platform, at which case I’d agree with you, no longer useful. Unfortunately for decades we’ve been quite a bit short of that end.
Just checked because I couldn’t remember exactly what OS info mine included last I looked. It’s quite generic: Mozilla/5.0 (X11; Linux x86_64; rv:128.0) Gecko/20100101 Firefox/128.0