Blockchain-based systems are... not simple, though they do do work. The advantage there is... what? The ability to verify sender/recipient? At the cost of massive power consumption to keep the encryption going, an unnecessary level of distribution for most purposes, and, as best as I can tell, the inability to delete anything that nobody involved cares about anymore.

If anything replaces email, it'll be basically email with security built in (end-to-end encryption and the guarantee that the From address matches the sender). More than that (maybe a little extra, I might have missed something) is overkill. Even that much is unlikely to come up except as patches on a per-provider basis, which might hopefully become omnipresent enough to remove support for the old version.