Proton has had a freemium model since 2016 and take no VC money. The key seem to be a free plan that is cheap to operate and has some features limitation to push subscriptions. Free plan has at most 1GB of drive including mails. And has Limited number of calendars, alliasses and mail tags.

Proton paid plan is around 10$ a month so it isn't cheap. But cheap isn't sustainable.

Source : https://www.youtube.com/watch?v=acWkkLaEsrU (Proton CEO interview)