I don’t think it will hurt them, because I think the majority of Proton users want exactly what you didn’t. There are lots of options for email using your domain, but I don’t know of any cloud suite providers that respect your privacy like Proton does.

Also, I am surprised that with the amount of different plans they offer none do what you wanted well. I thought they had a family plan for just mail without the other services, but they only have a business one and $7 per user is not a great price.