(Editor of the JMAP spec here.) We at FastMail are using standard IMAP/POP/SMTP, which we offer to all our customers to access their email. In addition, we have invested a large amount of time and resources in R&D to design a next-generation client-server sync protocol based on our deep experience with email. While we could just use this ourselves, we believe an open internet built on standards is healthiest for everyone in the long run. That's why, once we felt we had a solid foundation, we took this work to the IETF where a working group was formed. The spec has evolved considerably since then based on the feedback of many other experts in email from both the open-source and commercial world of email vendors.
The spec is now nearly complete and we expect standards-track RFCs to be published later this year. Adoption will then happen slowly over time, as it did with IMAP. Smaller, more nimble companies are likely to move first and then larger companies following later. IMAP took many years to get widespread adoption; I hope we can make the move to JMAP a bit faster, but these things always take time (often measured in years!) to develop, test and release.
I love XKCD as much as the next guy, and of course it's kind-of right, but also too cynical: the only other alternative is to not make an effort at all, and accept the status quo is the best you can hope for. We'd like to do better than that. If we fail, at least we know we tried.