E-mail is horrible for privacy, spam, instant messaging, etc. PGP “works” in very limited scenarios, and e-mail is not really one of them.
Plus these two statements seem unplausible for me:
we can assume you’re protected by PGP when writing to most users,
and
and with the added effect of not needing to convince anyone to install anything since from their end it’s just an email.
I disagree with the first statement, most users don’t know what PGP is and therefore don’t have keys, so you can’t encrypt anything to them. The only way most users would use PGP is if something sets it up for them, alá protonmail or my using some special client. Since you’ve said that from their end it is just an e-mail, how does Deltachat add any meaningful encryption?
AFAIK signal stil requires a phone numer for registration, however you now can add people by their username.