I always felt that the <!DOCTYPE html> simplification, while nice, is still a little weird because it immediately precedes an <html> anyway. I recall a blog years ago suggesting that HTML 5 should simply have said <html version=5>, and I think it would have been nice to see something that sane. :)
I'm sure the spec authors would love to make that simplification too, but one of their goals is compatibility with deployed browsers. <!DOCTYPE html> works to trigger standards mode in all currently-popular browsers.
Not only it triggers standards mode, but that's the sole purpose of this declaration in HTML5 (well, strictly speaking you might want to use it for something else, but that may not be the best idea).
If you serve your document with application/xhtml+xml you can omit it — this MIME type itself triggers standards mode so doctype becomes redundant.
No, omitting version is intentional (and version number did not mean much in the past). HTML5 spec defines parsing algorithm what should be used for all html versions and will never require browsers to do anything different for different versions of html.
If the future browser will be capable to parse HTML8 it will be able to parse HTML5 just fine.
I think the problem is that a 2009 browser will not be able to notice that the HTML8 it's seeing is potentially beyond what it can do. With versions, a check for versions above 5 could produce a message that says that some parts might not display properly.
