xslt was a stripped down dsssl in xml syntax.

dsssl was the scheme based domain specific "document style semantics and and specification language"

the syntax change was in the era of general lisp syntax bashing.

but to xml syntax? really? that was so surreal to me.

Also as someone else pointed out the same complaints that JSON Schema "isn't in the standard, it's a separate standard" apply to XSD. It is still a different standard even though during the height of XML mania it sometimes seemed like XSD was inseperable. XML did have DTD baked in, and maybe the author meant DTD in that section, but that was even worse than XSD (and again both were why RELAX NG happened).

XSD was (is) not so easy to adopt, but I don't agree that it's a monstrosity.

Schema are complicated. XSD is a response to that reality.

The XML ecosystem is messy. But people don't need to adopt everything. Ignore Relax-NG, ignore DTD, use namespaces sparingly, adopt conventions around NOT using attributes. It generally works quite well.

It's a challenge to get comfortable with XSD but once that happens, it's not a monstrosity. Similarly, XSLT. It requires a different way of thinking, and once you get that, you're productive.