I looked into adding JMAP support to Thunderbird but the client is so tied around the ideas and principles of IMAP, it needs surgical refactoring of many parts of it and I don’t love C++.
So instead in my spare time I am developing a JMAP only gnome email client, using many Stalwart libraries. Think Geary but Rust instead of Vala, GTK4 instead of GTK3 and JMAP instead of IMAP. It’s been mostly an excuse to play with Rust and gtk-rs and Relm4 (beautiful Elm inspired rust bindings for GTK4). Someday, it will be released.
Client support for a new protocol is never that quick, but I believe adoption will happen, at least outside of the big providers, who will never support it.