Buy a subscription and it all goes away!

Not really. Them not using ads means they can't make money that way, so the only way they can make money is through subscriptions. Even a not for profit needs to pay people and pay for overhead. If everybody used the free service they would go under over night. As long as they continue to respect privacy they have every right to have these pop-ups. Especially since they don't force you to subscribe.

And for a damn good reason... Companies need money to operate. Proton may not be for profit like Google, but they are not a charity either.

Very little is free without strings, what's been normalized (in a bad way) is the concept that you can have free things that don't intrude asking for money. That only happens in the venture capital "get em hooked" stage (and we've seen a lot of it because the Internet is still relatively young). Even KDE is now asking for money (granted once a year ... but your usage of their desktop doesn't require them to run expensive servers).

If anything, this reply shows how entitled some people are.