First they need to get massive email providers to offer JMAP support, and then get clients to support it. Surely this is simply too big a ship to steer.
Google is not going to support this, and isn't that >50% of the market?
They just need an IMAP/SMTP <-> JMAP bridge. This allows (small) providers to add support without much effort and there's suddenly a demand for clients (and vice versa good clients will increase demand for JMAP support).
The website lists jmap-proxy (https://github.com/jmapio/jmap-perl) Its age, technology choice and lack of documentation aren't exactly confidence inspiring, however.
Stalwart (https://github.com/stalwartlabs/imap-server/) seems to include a proxy feature (for both, though it's non-obvious to discover) and be better maintained.
A functioning proxy is make or break for me. If it works I can jump right into making a client, if it doesn't there's no point.