I've used Thunderbird exclusively for desktop mail for many years. To my knowledge, it still sucks less than any other free and open option, but that's not much of a bar.

Over time, Thunderbird has become slower and less reliable, most notably in the area of search. While the advanced search tools are excellent, the results are lousy. Email that I know exists is often unfindable until I force Thunderbird to re-index my whole mailbox.

Seeing F/OSS devs of (what I consider to be) a critical app like Thunderbird talk about "modernizing the interface" is the worst. The interface is fine and there are far bigger problems -- problems related to actual functionality as opposed to prettiness -- that desperately need work.

And to echo others' comments: courting the "average user" is worse than a waste of resources, it's an active turning away from the core users and supporters.