>If pronouncing Xochimilco according to English orthographic conventions is important to you as a matter of principle

No, not to me. I speak Spanish natively, but even I don't know how to say that. My first guess would be "Jochimilco", but I'd have to look it up (I'm not going to). I'm just saying that having Xs in weird places would not stop an English speaker from inventing a "wrong" pronunciation on the spot.

>But it’s a Mexican place name that has a canonical pronunciation that is not difficult for English speakers to approximate, so I can’t really see the point.

"Mexico" itself is also not difficult for English speakers to approximate, yet they don't. Clearly approximating the local pronunciation is not how foreign speakers decide how to pay toponyms, and that's fine. That's how languages are shaped.

My point is just that it makes no sense to get hung up on speakers not pronouncing loanwords "correctly". If we're going down this path, we should also complain that Spanish speakers write "fútbol" instead of "football", and that tea is called "tea" instead of "cha" and spelled "荼". We should demand that words be crystallized in their pronunciation and orthography when they cross language barriers.