There have been a couple of discussions here and elsewhere about it. My understanding is that Thunderbird is no longer a priority project for Mozilla Foundation and has no full-time developers paid by Mozilla, but it still uses Mozilla resources, and a number of volunteers continue to maintain it.
So, it's kinda in the category of Sylpheed or Claws, in that it no longer has major corporate sponsors, I guess. I think Evolution has a bit more corporate sponsorship (Red Hat, which has developers working full-time on it, while SuSE stopped funding its development a while back; I don't think Canonical contributes in any significant way, either), so if having a corporate developer behind the mail client is necessary, I guess Evolution is the only choice for Linux users.
Frankly, I wish Red Hat, and everybody else, would put its people on Thunderbird. It's far from perfect, but it's farther from awful than Evolution. But, I don't need Exchange integration, so I may be missing the big picture for corporate users.
They know they have to pivot to a more modern codebase, and have been discussing modernizing TB on a regular basis. Most of the traffic in April is about a "Proposal to start a new implementation of Thunderbird based on web technologies". Go read it!
A rewrite with a skeleton crew sounds like a disaster waiting to happen. Mozilla got a ground up rewrite, and even with big resources and a big team, it nearly killed Mozilla.
Then again, the tools are much better than ever. Building desktop apps can move very fast with Electron (or similar) and the like. Nonetheless, I think I'm on the pessimistic side of the argument about a total rewrite. I wish them well, but I suspect it'll fizzle out, as previous similar efforts have.
I'm not sure which Mozilla rewrite you're talking about, but in any case I feel that rewriting TB is doable. As you say there are better tools/frameworks, and the work done by asuth on the FirefoxOS mail client and glodastrophe (https://github.com/asutherland/glodastrophe) are good starting points.
Oh, you're in a for a treat! Jamie Zawinski wrote wonderfully on the topic of the rewrite of Netscape Navigator (e.g. Mozilla): https://www.jwz.org/gruntle/nomo.html
There have been other posts about it, but that's a reasonable place to start. Spolsky has also written about the dangers of rewrites, including Mozilla as one very public case study.
Interestingly, jwz also had a lot of things to say about the mail client that became Thunderbird, back when it was part of the Netscape suite.
There's a really long history with the Mozilla projects, and many lessons have had to be learned multiple times. I wonder if perhaps this is one of those occasions. Then again, the best thing that ever happened to Mozilla was Firefox, and that was one lone programmer deciding to (kinda) rewrite the damned thing to be lean and mean and scuttle all of the ancillary crap that Mozilla had accrued over the years. Maybe Thunderbird needs the same treatment...but, where's the Blake Ross to do it?
IIRC 5 years ago mozilla announced that thunderbird was not a priority anymore and considered dropping it.
Then 1 or 2 years ago mozilla announced that thunderbird sharing the firefox codebase was unnecessarily taxing firefox and the remaining thunderbird developers were struggling to follow the pace of firefox. Moreover that they believed thunderbird had little potential to have an impact as firefox has. So they proposed to separate thunderbird from firefox and cut the ties entirely.
Last news I heard was that Thunderbird could join seamonkey and become a community effort with some backing from mozilla[1].
