And Pubsubhubbub/WebSub is a plain simple upgrade on top of that model, for when you do not want to rely on polling only, e.g. because you want updates delivered quickly (vs polling, where most implementations sensibly have logic to adapt their polling speed to posting rate of a resource).

One of the benefits closed platforms have is that they can deliver posts inside the platform immediately, WebSub brings the option for that to feeds on the open web, without requiring subscribers to poll every <5 minutes and without requiring them to do large changes under the hood, e.g. introducing new non-HTTP protocols which can't be used on all hosting options.

For end-devices other update mechanisms are useful as you say, and systems speaking them could hook onto Websub hubs to get notifications they then translate. E.g. your typical wordpress blog has no chance of offering a XMPP channel, but it can ping a WebSub hub since it's only HTTP.

If you want to "upgrade" the old server-client model (request-response) to a realtime instant bidirectional information flow (dialogue communication), you surely want to look at protocols such as XMPP.

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.