I like the idea of being able to easily run your own node, but this sounds like a UX nightmare unless there's also some way to better manage relays.

I'm imagining having to add a relay for every single person I want to follow and it sounds tedious.

The other concern is discoverability. If many people are only posting to their own relays, I'd never find them unless they have a webpage or something that I happened to stumble across. And if they're posting to public relays, why bother having a private mirror (which may be incomplete in terms of replies)?

I get that it could serve as a backup, and I appreciate that aspect. The other benefit would be that it'd be easier for small communities to form like I could set up a server for my local Makerspace or HAM club or whatever.

Bottom line: limited value unless combined with other changes

I'm also suspending disbelief that anyone would take a one-time payment to host something forever. That's a huge economic problem that would need to be addressed to go from "thought experiment" to "implementation"