A Google Pixel is one of the easiest flashable devices, and one of the few devices you can securely use GrapheneOS with.
Regular people won’t do that though, protonmail isn’t worried about the savvy hacker scenario, but the simple consumer
You need to look at the percentage of people that have flashed their phones. The proton mail folks need a large addressable market that readily spends money, loves the convenience etc, so they can charge them and make a profit.
What I don’t get is, why can’t they simply charge their existing users 30% more - if they aren’t profitable.