Speaking of "correctness"... It seems to me people almost never mention that while schema verification can detect a lot of issues, in the end it cannot replace actual content validation. There are often arbitrarily complicated constraints on data that requires custom code to validate.

This is analogous to the ridiculous claim that type checking compilers can tell you whether the program is correct or not.

The impression I've got from the last 20 years is that a chunk of the XML community gave up on XSD and went to RELAX-NG instead, but only got halfway there.