They don't need the content anyway, just metadata.

ProtonMail uses PGP encryption to encrypt emails, which means your meta data, including subject line is vulnerable to data collection. Also there is no forward secrecy with current PGP standard. See quotes from below:

We have built Proton Mail with PGP fully integrated, ... All messages between Proton Mail users are automatically end-to-end encrypted.

https://proton.me/support/how-to-use-pgp

Subject lines and recipient/sender email addresses are encrypted but not end-to-end encrypted.

https://proton.me/support/proton-mail-encryption-explained

PGP (especially for email) exposes much more info to outside party than any good communication protocol, like the signal protocol or OMEMO used by XMPP.