Eh, I also thought that at one point (regarding XSLT 3.0), but it turns out that parts of it are an underspecified mess. E.g., you can specify static parameters to be supplied to the document, but it has nothing on when or how any type-conversion errors would be signaled [0]. And in general, it has very imperative features like shadow attributes that purists would turn up their noses at. I don't see it becoming widespread anytime soon.
[0] https://github.com/Paligo/xee/issues/32#issuecomment-2762343...