Thunderbird is not dying. Great work has already taken place by some talented developers. Need proof? Check it out (though take note it’s a beta and shouldn’t be used in a production environment, yada, yada yada). That said, it been a long road to 3.0 but progress is being made.
You can also take a look at Postbox (run by some ex-mozilla folks), based the same core code with some pretty innovative features.