I won't disagree with your summary of the situation with email, but I think I would add that it is possible to have federated reputation systems which would solve much of the spam problem with email, at least.

To give more detail about the sort of system I mean, here is an excellent description of the approach favoured by the team behind the Matrix communication protocol:

https://matrix.org/blog/2020/10/19/combating-abuse-in-matrix...