They need to motivate people to use FastMail to boost JMAP adoption and they've failed to do that for a decade.
They post their 12 days of Christmas blog every year and never a new feature to be found.
If you can't add shiny features to your service with a new shiny protocol that promises to break down silos, then it doesn't stand a chance.
What will replace IMAP will be what changes email forever, and we've not seen that yet in an open protocol.
We should revisit Google / Apache Wave