I have Proton Mail addresses using my domain.
Those are your addresses then not Proton's. Hence why switching is easy and is irrelevant to my complaint which is specific to the domains listed
Wikipedia says...
In economics, vendor lock-in, also known as proprietary lock-in or customer lock-in, makes a customer dependent on a vendor for products, unable to use another vendor without substantial switching costs.
If I want to switch away I have to pay every month in perpetuity to deliver emails to my new provider. In other words, I'll always have to be a customer