I have a similar sentiment towards Thunderbird. Every once in a while I try using other Linux desktop clients but I always go back to Thunderbird. Feature-wise I don't miss anything. My biggest problem are various long-standing bugs and occasional crashes. Each time it crashes I wonder whether that is something that might be exploitable by a specially-crafted email.
If I can wish for something, fix this bug from 2009 https://bugs.launchpad.net/ubuntu/+source/thunderbird/+bug/3... (but originally from 2007)