I came to Deltachat because I was looking for an e-mail client (for mobile Linux, encryption-capable). To me, the biggest selling point of Deltachat is actually compliance with open standards and interoperability.
Ease-of-use is second (there are lots of easy-to-use proprietary systems). Deltachat means I can swap encrypted mails with people who do not know what an encryption key is.
The chat format is a mild downside to me: it discourages long-form letters and encourages short chat messages. Which is fine, but I like long-form letters, so I’m not using DC for a lot of mails, especially if my correspondant does know what an encryption key is
. Many mailclients also now have more chatlike threaded views.
I think Deltachat could absorb the e-mail community if it had a client with a more traditional UI:
I also suspect that a traditional mailclient implementing free Chatmail accouts, plus SecureJoin and/or WKD, would see a strong increase in popularity. Though the entire DC backend offers much more than just that!