I really love Thunderbird; I've been using it for at least a decade now. There are definitely places where it still needs improvement, after years and years. While Betterbird seems exciting in theory, the attitude of the project leader (who it seems is a banned former contributor to Mozilla Thunderbird), and honestly even the arrogance in the name itself gives me pause. The issues laid out in this[0] email thread are…
Brilliant jerks almost always have a net negative impact on a project when you factor in their adverse effects on their teammates. The existence of a competing fork might be healthy, though. Having competition can make your priorities a lot clearer.
Vim is a highly successful project for decades. And one could correctly attribute much of that success to Bram Moolenaar's driving it.
And yet, once Neovim was created with a much "nicer" and broader leadership team, innovation has absolutely skyrocketed, where it's highly likely that Neovim has almost completely replaced Vim as the preferred editor for most Vim users who take the time to think about their editors, customize them, etc.
I wouldn't be surprised if neovim starts replacing vim in distributions within a couple of years.