Again, you exclude large parts of the web if you require something like XMPP. It totally makes sense to look into such things where it's viable, and you can bridge them with WebSub, but that was not in scope for pubsubhubbub (and WebSub is intentionally only pubsubhubbub with minor clarifications and cleanup).

Pubsubhubbub was a (relative) success because everyone doing RSS feeds could easily add it with their existing tech stack.

I will maybe sound harsh, but to me it looks like the Web (JavaScript?) kiddos wanted their own half-baked solution. As they only know about the Web, then they reinvent the wheel on top of the Web.

Anyway, if the proposal is useful to some people, then it won't do harm to have it in the public domain.

Pubsubhubbub is actively used on the open web for over a decade, from large centralized services to wordpress instances on random PHP shared hosts, has been used for at least parts of Wikipedia, ... It actually works. I'd say that's a success compared to theoretical XMPP based solutions.