I write emails, usually long form (several paragraphs). And of course, I read emails too. I don't use any chat apps, nor Slack nor Teams. I regret the tendency of chat apps to integrate email; the result is that my correspondents tend to reply to my considered emails with "LOL <smiley>" or something like that.
The IMAP spec is poor; it's ambiguous and unclear. The only way to build a reliable client is to test interworking against as many IMAP servers as you can find. A better protocol would be a good thing.
The SMTP spec is NOT poor. There's no need to replace SMTP. It works fine.
It's some time since I read an explanation of JMAP (with the article doesn't provide). As far as I'm aware, it provides roughly the same service, but using HTTP and JSON. I'm not a supporter of the "replace everything with JSON" movement; I don't see the point.
I'm not that keen on the "replace everything with HTTP" movement either; if you piggy-back your new protocol on an existing protocol, there's a risk that your new application will start dragging the underlying standard along with it, to the detriment of the original application the underlying protocol was designed for. I.e., it would be regrettable if JMAP resulted in pressure on the HTTP standard to start including features motivated by the requirements of mail retrieval.