Another probably even bigger reason people don't self-host email specifically is that practically all email coming from a residential IP is spam from botnets, so email providers routinely block residential IPs.

Yeah, exactly this. The natural disaster in North Carolina is a great example of how I envision this going very badly. When you self-host at home, you just can't have the same kind of redundancy that data centers have.

I don't think it's an obstacle that's absolutely insurmountable, but it feels like something where we would need to organize the entire Internet around solving problems like these. My personal preference would be to have devices act more independently. e.g. It's possible to sync your KeepassXC with SyncThing at which point any node is equal and thus only if you lose all of your devices simultaneously (e.g. including your mobile computer(s)) are you at risk of any serious trouble. (And it's easy to add new devices to back things up if you are especially worried about that.) I would like it if that sort of functionality could be generalized and integrated into software.

For something like e-mail, the only way I can envision this working is if any of your devices could act as a destination in the event of a serious outage. I suspect this would be possible to accomplish to some degree today, but it is probably made a lot harder by two independent problems (IPv4 exhaustion/not having directly routable IPs on devices, mobile devices "roaming" through different IP addresses) which force you to rely on some centralized infrastructure anyways (e.g. something like Tailscale Funnels.)

I for one welcome whoever wants to take on the challenge of making it possible to do reliable, durable self-hosting of all of my services without the pain. I would be an early adopter without question.