Self-ejecting panels on three sides of the website are a horrible user experience. Other than that... The most important thing is not the protocol, it's the gui. At the moment email's gui is horrible on all platforms without exception. If/when a decent gui appears, protocols will follow. Also, JMAP did reading can probably be just WebDAV?
GUIs cannot be built either before or during the specification of a protocol. In fact, this is the part that should have the least weight. There are important steps, such as system auditing or making design decisions (like certificate rotation), that are not addressed. However, the goal is not to replace SMTP/IMAP, but rather to show how modern tools, when properly assembled, can replace older protocols with serious…
One has awesome certificate rotation and crap UI.
One has crap certificate rotation and awesome UI.
Which one will take over the world, and which one will only ever have 10 users?