JMAP doesn’t seem to have wider provider support (Fastmail, which created JMAP, seems to be the only one supporting it). It also has some pieces (IIRC, calendar, contacts) that are still being defined. Thunderbird supporting JMAP isn’t going to do much for Thunderbird.
On the other hand, if Thunderbird supported Microsoft Exchange natively (without extensions) for mails and calendars using EWS (deprecated) or MS Graph, that could increase its use in the enterprise manifold. From what I’ve seen, companies using Microsoft365 have been turning off IMAP (and POP), pushing employees to Outlook as the only option.
On the other hand, if Thunderbird supported Microsoft Exchange natively (without extensions) for mails and calendars using EWS (deprecated) or MS Graph, that could increase its use in the enterprise manifold. From what I’ve seen, companies using Microsoft365 have been turning off IMAP (and POP), pushing employees to Outlook as the only option.