"I'm not even aware of any proper alternative"
Exactly. I hadn't thought about it much until now, but if Thunderbird goes away, what will I use? I can't think of a similar quality product that meets my needs. The only people who aren't seriously concerned about the future of Thunderbird either are Outlook users (*cough* *cough*) or people who don't know there is a way to do email without HTTP (or what HTTP, IMAP, and POP even are for that matter)