Somebody no longer valuing what ProtonMail provides is not mutually exclusive with thinking ProtonMail could have done differently. I expect many ex-ProtonMail customers realize ProtonMail could not have done differently, but nevertheless no longer value the service ProtonMail provides. How could this be? It's simple: They previously had mistaken beliefs (namely, that ProtonMail could have done differently), and decided to no longer be customers of ProtonMail when the truth became clear to them.
If you sell me electricity from your cold fusion power plant, I am going to cancel my subscription as soon as I figure out that cold fusion isn't possible and you were selling me coal power all along. That I now realize it was impossible for you to sell me cold fusion power in the first place doesn't change the fact that I no longer see any value in doing business with your power company.