> One niceish thing about WebDAV/CalDAV is it's pretty set in stone for now.

I don't know if you've ever heard "Latin is a dead language"; many people think that statement is a somewhat negative-sentiment one, amounting to something along the lines of "there's no good reason to learn Latin, it's dead", but I've heard that it's actually supposed to be a positive-sentiment statement, something like "we can have confidence that contemporary interpretations of this text haven't changed in the last ~1800 years because the language itself stopped changing around then".