My grievances with Proton

I’m not sure if “vendor lock in" is quite the right term for what you mean. You already commited to vendor lock in when you chose to use a @protonmail address instead of bringing your own custom domain. But it’s a subscription lock in, sure. Overall I understand your point that it’s lame for Proton to gate features without a direct cost behind a subscription. I tend to agree. Though in this case I’m not sure what the solution is. You could apply the same argument to most of the paid plan features: custom domains, unlimited labels/folders/filters, IMAP/SMTP, etc. Does removing access to those features save them money? Not exactly. (Except storage space - that has an obvious direct cost.) But at the end of the day, developing and supporting all of these features does cost money. I like the idea of a grace period where you can still access the aliases, because it is a huge problem if you didn’t migrate any accounts from that email alias. Or maybe the aliases become receive-only in a limited kind of way.