Probably true but sadly there are equivalents to a user-agent string in a lot of technologies and they will never really go away because of compatibility constr

Probably true but sadly there are equivalents to a user-agent string in a lot of technologies and they will never really go away because of compatibility constraints. Entire operating systems contain hacks to work around unfortunately-designed-but-popular applications doing things they shouldn't. Device drivers too. Even the x86 processor specification in some cases is more about "what companies actually do" than "what some document says", because that is the only way things work correctly.