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.
Quick note, Proton AG itself (the for profit company) still owns, operates and develops the Proton services as we know. The only difference with the non-profit structure is that Proton AG is owned by the proton foundation. Which basically is a protection against aggressive takeovers and the enshittification that would follow. Also, tax advantages, probably.
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).
It doesn't. I didn't say that proton can't show me ads or it must serve me for free. But the fact that you are attacking me personally for asking a question about a product in a community dedicated to this product clearly confirms that my previous reply was spot on.