That would be nice to have, but it's less realistic.
Some clients already speak proper HTTP, and others can too, with little modification. For modified RSS like this, you have to make a standard first, then push both servers and clients to use it.
It also requires a "smart" server, whereas you can serve your atom or rss feed statically with a standard HTTP server handling caching and conditional requests essentially free.
Yeah, although dumb servers could just return last N posts as usual. It's one more case to handle on the client, though. (The alternative is breaking compatibility with older feeds, which is way worse IMO.)
That would be nice to have, but it's less realistic.
Some clients already speak proper HTTP, and others can too, with little modification. For modified RSS like this, you have to make a standard first, then push both servers and clients to use it.