literally this. user agents are an obsolete feature (i considered it obsolete around 10-15 years ago) and whenever a new protocol includes it you already know its made by backseat networking "engineers". all user agents do is break more shit because people think they're smart or clever for modulating their machine behavior based on them (blocking, working around broken standards, etc)
"there exists a use case that wouldn't be possible to implement without user agents" is not an argument. it's not philosophically valid. its dead obvious that you do not consider the whole picture if you make this argument. why does that 0.1% use case matter? especially given that it's not officially supported. especially given that for the last 10 years, the likes of google have shoved everything _straight_ into the web specs when they started considering it an official supported use case? if you are doing something that's not an official supported use case, why do we need to have hacks to support it? obviously the least ecologically harmful solution is for your use case to not be supported. of course we are talking web here, where scope creep is infinite and the protocols do nothing well instead of one thing well.
this is the very problem with the web, that "it doesnt have a concrete use case maaaaan", "ill just add and take what i want as i go", "the web is just the web maaaan you need to think on my wavelength to get it". these people embedded in web actually unironically just make shit up as they go along. they literally operate on an oscillating wave where today feature set 1 is good, tomorrow feature set 2 is good, and the day after, (what is essentially) feature set 1 is good. arguing to have a user agent string is the same idea. this is in strict contrast to a well designed product that actually solves a specific problem, like Standard ML, or a good SQL implementation (disregarding the specification conundrum) or JSON or VGA or TCP.