It literally was abandonware for several years.
It was only recently resurrected by 1-2 devs a few years ago and they’ve had to rebuild the community, funding and contributor support.
The fact that Thunderbird worked perfectly well even while being abandonware is a testament to Thunderbird and email.
I think the last part they really need to get completed is separation from the Firefox code. We’re already seeing accelerating delivery of features which should hopefully improve.