Clearly that's not the use you have for a new XML library. It's a use you're imagining somebody else would have for it. And because you're just imagining the use case, you've failed to think through what "good" would actually mean in that use case.

To replace libxml2 across these ecosystems you would need it to be API-, ABI, and probably bug-compatible with a decrepit old C library. That's not something anyone or anything can write from just the XML spec.