The difference is that, any time you check your email, a basic TLS MitM could exfiltrate your entire inbox without your or ProtonMail's knowledge — exactly the

The difference is that, any time you check your email, a basic TLS MitM could exfiltrate your entire inbox without your or ProtonMail's knowledge — exactly the same threat model as Gmail or any other webmail provider. Further to that, I'd trust Google's infrastructure to withstand compromise much more than I'd trust a datacenter run by a small company that I know much less about. (Tinfoil hat: sure, I have to assume that the NSA has a copy of my Gmail inbox, but god knows who else may have owned ProtonMail.)