Tim Bray, who is one of the editors of the XML standard, and creator of the awesome Annotated XML Specification, writes about the absurd wart of <!DOCTYPE>: Drop the <!DOCTYPE>. I've long thought that of all the things that are ugly about XML, <!DOCTYPE> is the worst: it could easily have been separated out into another standard, giving competing validation techniques an equal chance, and saving us from carrying it around forever in the spec.
It turns out Tim and Norman Walsh (another XML luminary) have each written speculative follow-on XML specs: Norman's XML 2.0, and Tim's XML-SW, both of which omit <!DOCTYPE>. Neither of these is being worked on as an actual standard, but it's interesting to see where people think XML can go. (I know: "Hey buddy! I'll tell you where XML can go!", ba-dum bum!)
Tim's a smart guy, though. He observes (correctly):
I suspect that nothing will happen; we’ll go forward indefinitely into the future, with XML implementors required to read the XML spec, learn to ignore all the Doctype stuff, then go read the Namespaces and Infoset specs, and synthesize all that in their heads.
Standards evolve to a point where they are good enough, and they get a large enough adoption, then they stop, because it isn't worth the marginal improvement to leave all the earlier adopters behind. It happened with HTML, and it's happened with XML.