Realistically, permanently walking away from K-9 Mail never was an option in the past. There was no one else to take over as a maintainer. It would have most likely meant the end of the project. K-9 Mail was always too important to me to let that happen.
But this is no longer an issue. There’s now a whole team behind the project and me stepping away no longer is an existential threat to the app.
Good on Mozilla for helping Thunderbird continue!