You shouldn’t refuse to use a service because they might change their mind one day.
Sadly, when it comes to privacy, yes you should. Because privacy today does not equal privacy tomorrow. You can't magic all your data out of their systems if things change. That's the issue. I can take their privacy promises today, but if those promises can be changed on a whim by new owners... what good is that privacy promise? It isn't.
Proton sells privacy. A privacy that can be rescinded.
Unless you expect everyone to roll their own everything.
I know it's kind of an elitist attitude, but yeah, I fucking do. If they're really concerned about their privacy.
Ed Snowden didn't roll up to Glenn Greenwald and go "Yeah, just use any off-the-shelf messaging service that says its private." No, he made Greenwald roll up his own.