mm is convention for ObjC++ files, which is a required dialect if you need to use most the platform libs on Apple platforms.

Being the standard extensions for the two most major platforms, I wouldn’t call them rare at all.

And which further reinforces my point. They could have mandated an extension BUT it would have been going against convention which makes it decidedly not easy. There’s not one singular team in charge and there’s lots of friction in such a seemingly minor change.

I’d argue too that even with modules there’s very minimal benefit to a standard extension. Any such benefit would have had even greater benefit for the rest of the language.