> I don't know of any other protocol which is replacing this

JMAP: https://jmap.io/spec.html.

The core and mail specifications are where the focus is, and they’re what’s currently approaching standardisation in the IETF, but there are also the calendar and contact specifications in progress, which are designed to replace CalDAV and CardDAV. I don’t think they’re getting a great deal of attention now, while core and mail are finalised, but once those are done I expect more attention will come to them. (In the mean time, I believe we’ll be speaking the calendar and contact JMAP drafts in FastMail when we switch over to JMAP.)

The ability to do everything over one protocol instead of at least four—IMAP, SMTP, CalDAV and CardDAV—is one of the nice things about JMAP.