I self-host email, and after spending weeks dealing with a very persistent asshole trying to break in to my systems, was looking at options a while back. (I still self host email.)
Proton was the first one I looked at, but they charge per-email address, including aliases, which is a blocker for me. (I use unique email addresses for each service I use, and more for other things.)
But this is even worse. I would never use a service that would start sending my email to someone else if I stop paying, that's insane.
There is no way Proton is anywhere close to namespace saturation. The big mail hosters have orders of magnitude more addresses behind single domains.