> unless the recipient is also using protonmail

Or any similar service from another vendor? Or hosts their own email. If someone using Protonmail emails me, their data is also not getting sold for example, it's just stored on my laptop

Even if it's another self hosted service, proton still needs the plain text in order to send it to them with transit encryption only.

Proton does have interoperability with PGP/GPG but very few people use that because of its UX.