the whole point of deltachat is that it is reusing an already standardized protocol with existing servers.
i am using element/matrix and i have tried briar. the usability of deltachat and the ease of onboarding beats both of those. briar was especially difficult to get started with and only has a very limited usefulness compared to the others. and matrix is simply very complex and easier to misconfigure.
Briar had trade-offs, for example, it is not available for desktop. I do not have use for Briar, personally. I use the rest, but Briar is worth a mention.
forward secrecy is independent of the transport protocol. it's only dependent on the encryption. messages encrypted with forward secrey can still be sent over SMTP. deltachat devs are working on that.
signal does not use a standardized protocol, and it requires a phone. that's not an alternative. my children have deltachat on their laptop. i can talk to them when i am not at home without needing to give them a phone.
Why implement something PGP-like, without forward secrecy, 13 years later, beats my understanding. I mean, 13 years is also the time difference between OTR and PGP. I guess some devs don't read cornerstone papers of the field they supposedly specialize in :)
Yes, I think the deltachat people should have gone with a different (still open source of course) encryption method that supports forward secrecy, and not try to be compatible with encrypted emailing. You can still use the email server/client infrastructure, but don't try to serve "normal" emails on the same system, and don't allow normal email providers to take part.
i am using element/matrix and i have tried briar. the usability of deltachat and the ease of onboarding beats both of those. briar was especially difficult to get started with and only has a very limited usefulness compared to the others. and matrix is simply very complex and easier to misconfigure.