"ProtonMail uses Zero Knowledge Encryption, which means it is technically impossible for us to decrypt user messages" is kind of false within the definition of

"ProtonMail uses Zero Knowledge Encryption, which means it is technically impossible for us to decrypt user messages" is kind of false within the definition of "impossible". It is exactly as feasible for them to crack encryption as it is for any other party with an encryption and security background, so really this is "The only way for ProtonMail to read your email is by cracking it, which can be prohibitively time consuming" but really it won't be because the fact that "ProtonMail takes care of the security" means that if the service ever becomes mainstream popular, the proportion of people using easier-to-crack-than-should-reasonably-be-the-case password will skyrocket.