Because, you know, the day they stop committing, is the day that thunderbird stops working and takes all your mail store with it.
Oh, actually, that's not true! I'm still using TB3 installed with my Ubuntu 10.10, and there's nothing missing there. I'm sure that the latest 2013 TB is going to be usable through 2015 at least. No need to look for replacement now.
Kindof glossing over the issue, no?