I think you’re confusing “encrypted in transit” versus “end to end encryption”
Your email between protonmail and gmail would be encrypted between the servers, but both protonmail and gmail could read your email. OpenPGP makes it so incoming emails are encrypted with your public key, so there’s an extra stage of encryption that only your email client can decipher. OpenPGP therefore has the capacity for end to end encryption
It’s not like we’ll not see it coming Once threads.net starts federating with the big instances, it’s all over
The information that can be gathered and sold is pretty intense, even when your account is on another instance