But you did mention the motivation of geoip for detecting location for selecting a language, and given that's what people still do these days despite Accept-Language headers being available for most clients, I think it's worth a call out.
I'll also say that even though I invented IP location technology, I am an avid user of privacy-shielding VPNs. Just because you invent something doesn't mean you don't regret some of the uses of said thing.
Accept-Language headers aren't obviously the user's preference, though. There's the four users that manually set it, and for them it is their preference. Mostly everyone else is getting a default based on their OS language, which often isn't really something they set either.
There's also the problem where the country appropriate website for where you are may not have a language you understand.