The cost of maintaining an existing alias is actually, factually, absolutely zero. Maybe epsilon, if you want to get pedantic.
It does not cost Proton any money to “host” an alias. Let me explain this way: Proton provides users with infinite +suffix aliases that can be appended to your Proton username. These aliases all forward to your base Proton address, and they are backed by the same infrastructure as a Proton alias. Since Proton allows any user, free or otherwise, to then have infinite +suffix aliases, imagine if their costs were nonzero. What would that look like? Proton would have an infinite, unbounded potential cost and that would surely be a catastrophic gap in their business model.
And I say “hosting” in quotation marks, because hosting an email alias is a categorical error, because there’s nothing to “hosted”. Hosting would imply continuously expending resources to maintain a service, but an alias, to an email provider, is just a row Proton’s database. This row costs nothing to “maintain”. Even if named aliases require a discrete record where +suffix doesn’t, the cost of that record once it already exists is still effectively zero. Storage for a few bytes of text is not a meaningful business cost, especially when Proton’s policy is to never reclaim those aliases anyway.
This also reflects a common misconception about how email aliases actually work. An email alias, when provided by your own email service, is not like an additional inbox with its own storage and its own dedicated reservations for a user. It’s literally just a name that’s reserved for you and maps into the same inbox.
This is a false comparison. These aliases providers cannot be compared to Proton email aliases, because the technical structure of an email aliasing service is completely different from that of an email provider offering aliases for their own service.
A provider like Addy has to maintain an independent mail server that acts as a proxy between the user’s email provider, which is where their costs incur. Services that are dedicated to providing aliases all exist in this nature. Their independent email server receives emails on behalf of your email provider, then forwards them to them. They cannot encourage indefinite, unfettered free usage of their service because they would invite the entire world to route their emails by their proxy service, which would create unbearable load in their routing infrastructure.
In the end, there is only the unnecessary restriction of the user of something that is costless on top of what is already provided. The alias is already registered under the Premium plan, and it cannot be reallocated. It will forever, indefinitely, be registered as aliases cannot be reclaimed under policy. The restriction post-facto is actually entirely artificial, and in the spirit of my original point, a form of vendor lock-in.
