JMAP and friends are very niche, none of the "mainstream" email clients (that ship with most computers/phones) support it. So this feature being available is unlikely to grow the userbase, IMO.
Now JMAP is quite a bit nicer to use than IMAP's API, but IMAP's gravitational field is too strong to be supplanted. IMAP is also becoming somewhat of a niche protocol, as the majority of users use vendor proprietary protocols for accessing their emails on Gmail, Outlook/Hotmail, etc. So why invest the time to add a niche replacement for IMAP when the entire protocol is a second class citizen to mainstream email clients.
If you want to push a new technology, you need to start somewhere. That's exactly what's happening with JMAP. It was created by Fastmail to use as a bridge between their servers and their own apps a case for which popularity doesn't matter. It's basically a modern vendor proprietary protocol but done in the open.
From there, support is only a matter of someone being interested enough to implement it and manifestly it's working. There are now three servers (Apache James, Cyrus and Stalwart) and some clients.
Now JMAP is quite a bit nicer to use than IMAP's API, but IMAP's gravitational field is too strong to be supplanted. IMAP is also becoming somewhat of a niche protocol, as the majority of users use vendor proprietary protocols for accessing their emails on Gmail, Outlook/Hotmail, etc. So why invest the time to add a niche replacement for IMAP when the entire protocol is a second class citizen to mainstream email clients.