I'm glad this is happening.
My gf's father has a 2013 Apple computer that's absurdly slow. His only reason not to switch to a libre system is email. He tried Thunderbird and found it ugly and barely intuitive. I was hoping something like this UI rebuilding would happen, because he is still willing to wait to see how options change over time.
M1 incompatibility with an old program he uses stops him from getting a Mac. He doesn't even consider Windows because he dislikes it… Thunderbird's ugly interface stops him from getting a libre desktop system. I'm hoping Thunderbird delivers their new polished and intuitive interface before his old program gets replaced by something he can use in an M1.