I get that this is a huge effort and that it takes a long time for protocol adoption (especially if the big companies aren’t behind it). I don’t know why Fastmail has developed mobile apps but no desktop applications so far. One justification could be higher usage on mobile, but it still leaves desktop users out (of JMAP).
Without more email clients supporting it, mail providers don’t have any incentive to support JMAP. Mozilla Thunderbird started looking at JMAP but hasn’t progressed on that all these years.
> In 10 years time, I hope to post about how Cyrus and JMAP have taken over the world
I believe the biggest hurdles are other email clients and providers not adopting it. The biggest threat is Microsoft, as usual, pushing its own protocols and client and using FUD to brainwash CIOs into believing that any protocol outside its own is a major security threat that just cannot be handled.
If either Apple or Google could be convinced to implement and support JMAP, this could take off a lot faster.