And why won't that be the configuration ActivityPub converges to? Why will people be signing up for random group instances rather than just pushing a button to get their own? That's what happened with blogs. Why won't it happen here?
Because the client-server paradigm means than an instance can’t be hosted by a mobile client (and even if it could, the system relying on DNS or a static IP wouldn’t work as most mobile IPs are dynamic) and thus should be provided by someone at a loss (or other user-hostile business model such as advertising if not outright extortion). The system needs to be as easy as Facebook/Instagram/Twitter if it wants to succeed. Having to manage a domain or a (stateful) server is a non-starter.
RSS doesn’t have this problem because it doesn’t require the client to have a static hostname, thus mobile RSS clients are totally possible.
This doesn't make much sense. I agree: the instance won't be mobile. It'll be hosted, the same way Blogger was. It'll be provided with the same economics as Blogger. Manage a domain? Most people will be "theirdumbname.mastohost.com". For like 20 different values of "mastohost". Those "mastohost.coms" will make all their money on the subset of people who will provide a credit card to be "theirdumbname.com".
People are sleeping on how well the blogosphere worked, and how much Twitter and Facebook were able to divert it purely through a catchy UX. The blogosphere now has that UX: it's called Mastodon. Meanwhile, Twitter and Facebook are collapsing, because the centralized social network is inherently unstable and susceptible to gaming.
