The two apps are identical and built from the same codebase anyway. K-9 is just a branding asset swap.
I’ve seen conflicting info from Thunderbird devs on how long they actually intend to keep both branding packages active. I’ve heard no longer than a year. I’ve heard only as long as it takes to get Thunderbird out of beta. I’ve heard they have some sort of agreement with FDroid that obligates them to keep it listed for some minimum duration of time (???). I’ve most recently heard indefinitely, because their build script is just a toggle now and it costs them nothing. Which one do I believe? I have no idea. I doubt K-9 will be kept around in perpetuity, though.
Choice is an irritating speed bump to people who don’t care to choose, which unfortunately is most of them.