ISPs are at a different level of the stack and already have an oligopoly.
ISPs and Instances both offer you access to a wider network. That one exists on a network level is another matter. If there were a multitude of ISPs, like there was in the dialup era, would you have wanted them to decide what domains resolve?
its very difficult to selfhost without permission from them lest you get marked as spam
That’s because they’re essentially defederating entities they don’t trust; exactly what’s being proposed here. The solution to defederation is not pre-emptive defederation.
What email is really suffering from is a failure of the network to combat abuse. That’s a real problem for the Fediverse too, because there’s almost nothing that stops someone from spinning up infinite numbers of instances and spamming other instances.