I think Thunderbird is just slowing down, even only doing bug fixes and critical updates users will still continue to use it. I guess when the time comes they would be looking for something in active development.

You might have heard that Mozilla announced to quit the further development on Thunderbird. In my eyes, this would be the right opportunity for David to do some marketing activity (blog entries, etc.) to tell the people that there is a vital alternative in place and regain market share. In my perception, there is still a big need of desktop clients for private users and small businesses.