What is rssCloud exactly and why should a long-time blogger care about it? I ask this not to be facetious. It's because I read your blog post with interest, but by the end, I still wasn't sure. Your second paragraph, which defines rssCloud, gave a kind of technical description (the "what") but not the user benefit description (the "why"). Unfortunately I found the Wikipedia article to be much the same. Basically, I couldn't really figure out "what's in it for me". For context, I'm a long-time programmer, I have blogged at amontalenti.com since ~2004, via WordPress since ~2005, and I even work for Automattic. So, if I'm confused, I bet a lot of people with less context are even moreso.
The main benefit of rssCloud is quicker notification of updates versus polling every 10 to 60 minutes, as most feed readers do. The drawbacks are that the tool generating the RSS feed needs to include another element (the cloud element), the tool needs to inform a server that an update has occurred (need a server), and there has to be an app that registers with the server to get updates, and to display those updates (need an app that supports rssCloud). Without all of those 3 pieces, rssCloud is not going to provide the value of quicker notification. You could also consult your fellow employee, Joseph Scott, who worked on the WordPress plugin, for more context.
I am not familiar with what is needed to support WebSub, but I assume there is some work involved, just as there would be for XMPP, Jabber and any other protocol that involves more than polling. That "extra work" is the barrier to entry that helps keep people using Twitter and Facebook - the users get that notification "hit" and network effect without any extra work on their part.
Thanks, that makes sense. So the core problem it solves is preventing the need for polling for feed consumers and apps, allowing for real-time notifications of updates. In theory, people could use rssCloud to adapt blogging platforms into microblogging use cases, e.g. using the simplicity of an analogy, Mastodon use cases built atop WordPress infrastructure.