When was the last time you actually wanted to do that, rather than just wanting to hypothetically raise that possibility for the sake of an argument about web technology?

Because in reality, based on my experience at least, no one actually needs that. Folks can copy a formula that they got from an image into wolfram alpha just fine. And the folks who can't don't actually benefit from MathML: they benefit from the JS Selection and Range functions, when site owners take the time to make sure that text-selection of a formula image leads to a LaTeX formula being put in the clipboard, instead.

"That's way more work" but since we're all using build systems anyway: no it's not. Write once, thousands if not hundreds of thousands of users benefit. The end.