While I don't have any data on it. I think that retention tactics that drag out the process might be more harmful in the future. Especially in terms of brand image. I might be cancelling the service for a short while or I might be switching companies/priorities. If the cancellation process is tedious, I won't feel like returning to that service.
In terms of the argument here, it feels like Proton Mail is trying to mirror Spotify's argument. The only problem is that, Spotify was a direct competitor to Apple Music. This is entirely different. Apple is providing a marketplace for apps, and many companies like Proton Mail are actually exploiting the system.
The only fault here is Apple's approach to the issue. But in this specific case, I don't think there's any mafia action going on unlike the whole Spotify drama.