Nothing technical -- it's mostly a cultural thing. Backwards
compatibility is a religion on the net and the web.
And even in a production software environment, you can't really remove
features, no matter how slowly or how much warning you give.
Well... you _can_, but you'd better have a DARNED GOOD REASON.
This brings us to...
>The idea is to have such a better new way to do things that people stop
>using the old (deprecated) way for all newly created documents,
I frankly don't see what's wrong with <ul>, <ol>, and <dl>. The users
grok, the software groks, everything's hunky-dory. These are distinct
communications idioms. In language design, orthogonality should only
be taken so far. It's good language design to optimize for common
idiomis -- especially in a document markup language, as oppose to
a programming language.
Consider the cost of migrating from the current idioms to your
proposed idioms: all that software has to get fixed (no big deal), we
have to hammer out the spec, all those HTML introductions, books,
tutorials, references, test suites, ... and finally: all those PEOPLE
have to learn the new idiom. (Studies show it takes 3x as long
to learn something if you know already know it in a contradicting
form as if you're learning it fresh. "Can't teach an old dog new
tricks," I guess.)
And for all that hassle, what do they get? As far as I can tell,
exactly the same expressive power as they already had.
Sorry, I just don't think it's worth the bother.