As Proton user,
I want my username to be completely separate from my email address and aliases
So that bad actors and others that attempt to email any proton domain containing my username will automatically get a 500 5.1.1 Address does not exist and that bad actors who use a list of email addresses to get into my proton account won't be able to because the email addresses they have have nothing to do with my username

This will solve two problems. So in future when you sign up for proton you first create a username, thereafter you can register as may email aliases as your plan allows against that username; so all email and so on is associated with that username from an encryption perspective, but you are not locked into any specific email address; and if you choose to downgrade to a free tier you simply need to choose one email address alias you still want to keep using and the others, still tied to your username simply become un-useable until you pay for a plan again.