You don’t need to self host email, Matrix or XMPP to use E2EE. I meant self hosting the web clients.

Nobody does that

HSTS, Certificate Pinning, …

HSTS is great but doesn't protect you against maliciously issued certificates, and Certificate pinning is deprecated on the Web.

Yeah, Open Source software down to the OS itself is important for security. But even then, who audits their own software? It’s probably 0.01% of the 0.01% of the general population you mentioned.

That's why you stick to software under high scrutiny and highly visible for security sensible stuff, and avoid using software with a broken security model for sensible stuff.