Presumably the value of building on existing not-particularly-well-loved protocols would be that only a small part of the stack needs to be extended. If you have to extend mail servers to support discoverable multicast endpoints, at that point you might as well just make a new protocol from scratch.

Another big problem with push, besides the overhead, is that everything is transient. If you aren't subscribed to a source when it sends something out, you will never see that thing, even if you subscribe later. That might possibly be acceptable for status updates, but definitely not for blog posts, photos, and other permanent content.

Of course, there are various request or sync schemes that could address this problem but anything like that would push this whole idea into Rube Goldberg territory as far as I'm concerned.