But that's the thing, browsers regularly send Accept-Language, if it's overall less accurate than ip geolocation, then it's hard to prefer it.
The worst case is pages displaying in the same language as the rest of my computer.
I mean, there are reasonable arguments either way here IMO and without actual data, e.g. how often users change their language from one or the other, it's hard to know what is better in the long tail.
And obviously users should be able to change their preference if we get it wrong.
Why would these people be badly served by being sent pages corresponding to their OS language? It's clearly preferable for them over poor localization.