It’s unlikely but not impossible. I’ve been using PM with a custom domain for about five years now, and never thought too hard about leaving.
In an ideal world, a company like ProtonMail would be cooperatively owned by the workers and paying users, sort of like a credit union.
Pragmatically, they’ve done fine stewardship of the service for the last decade or so they’ve been around. A big part of it is that their value proposition depends on stability and trust. But it could be better.