I am a Thunderbird community contributor. I can confirm that TB is not dead. Mozilla is only going to reshuffle the release cycle again and reorganize the paid stuff working on TB.
For those that are happy with current TB features: you will continue to use TB as always and there will be periodic stability/security releases as usual.
For those that want new features: paid mozilla stuff will work a bit less on new features, but they are now more open to new features contributed from the community. AND there are already some big features done, that are not fully exposed, like maildir-like message store (instead of mbox), sending of big attachments via file-storage services, Instant messaging integration, Australis theme changes, etc. You can expect that in stable releases soon.
And even if the community will not be able to produce big new features, it can at least do bugfixing and nice polishing. Many people welcome this as they are only bothered by the regressions big features bring with them.
Why not fork Thunderbird already?
There is already a group of developers that cares about the project. Thunderbird is not only developed by Mozilla staff, but also by community developers. I don't understand which advantages a fork would have instead of just developing it further without the Mozilla developers.
Where is this mythical group of developers that care about Thunderbird more than the existing ones do?
If they were happy committing to Thunderbird before, I don't see why they wouldn't continue. And i doubt that there will suddenly be an influx of fresh blood that wants to move it for some reason.
If anything, Mozilla has stated that this move makes them more open to outside contributions than they were before - making it more friendly for 3rd party devs and less likely they would need to fork it.
As has already been mentioned, this is a GOOD THING.
If it is community driven and supported, it means that it will be built to match community's NEEDS, not mozilla's needs and what they only BELIEVE to be their customers' needs.