UI is the part of software most prone to fashion. Fashions go in and out, and UIs and their usability goes in and out with them. I would rather stick to a usable, well-known UI than have TB be trendy and strictly worse, like the "flat everything" fad, or the "you don't need options" fad, or the "header bar" fad.
Edit: Taking a further look into the history, it looks like the project has been under a lot of turmoil WRT shutting down and being revived. Explains why I have avoided it for so long.
I don't even use IMAP, nor calendar. I download from POP3 servers and use the calendar on my phone instead.
If they get to a point where Android and Desktop Thunderbird have email, contacts and calendars synced, with a UI that's as nice as this mock it will be fantastic.
The type of person who cares about that will not be using it anyway. Trying to be hip is how firefox when from 30% market share to 3%. Stop going after people who don't care but are very loud.