XML without DTDs is a very reasonable subset that eliminates significant complexity (no need for an HTTP client!) and security risks (no custom character entities that are infinitely recursive or read /etc/passwd!) and would probably still work for >80% of users.

(I wrote such an XML parser a long time ago.)

Why throw out numbers when we all know you haven't actually measured that it's >80%?

In any case, the tooling around XML (DTDs, XPath, XSLT, etc.) is the reason to use it. I would go so far as to say the (supposed) >80% not using those features shouldn't have used XML in the first place.