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 design flaws. JMAP is flexible, mature, and easy to connect with current technologies. It's a better option.
> At the moment email's gui is horrible on all platforms without exception.
This is obviously a matter of opinion. Of course there are endless different email GUIs, on all the platforms; if you don't like one, there are many alternatives.
I cannot imagine that a "decent GUI" by modern web-development standards could be anything I'd prefer to what already exists. Email works and generally doesn't let designer ego get in the way.
> At the moment email's gui is horrible on all platforms without exception.
How can you say that when it's one of the benefits of email that it's an open protocol and there's thousands of different apps built on top of it. Terminal UIs, web interfaces, chat interfaces etc. - that should be one of the cases where there's a GUI for anyone.
Why do you find the panel experience so bad?
It's very annoying on mobile, covering half of the page all the time
When you scroll down, they should all disappear. Do you have JavaScript disabled? What browser and device are you using?
FWIW, I'm on desktop but I do have JS disabled (more precisely, I use NoScript).
This results in the top and left-side bars staying decorative and unobtrusive, but the concurrent visitor counter thingy is floating well above the bottom-right of the page. But then, as I shrink in the page width, the left-side bar and the concurrent visitor counter swap sides, and also a bunch of the content from the top is added to a new bar at the bottom. Eventually, the horizontal space between the side components becomes irrelevant such that they function as an additional bottom bar. At which point, maybe 40% of the vertical space is used for navigation and other shiny toys. This is really annoying, and the overall experience is inconsistent across widths.
(Also, at all widths, the top-bar has a transparent background, so main page text interferes with top-bar text as it scrolls.)
All of this is done in CSS, so presumably it can be fixed that way too.