Hold on there friend. I use Thunderbird on desktop and have used it for nearly two decades across Windows, Mac, and Linux. I love it! It's extensible, has excellent plugins that Outlook will never have, and I can synchronize my settings across computers and it Just Works. Also using Outlook on my work machine takes a second to delete emails and it does all sort of weird behaviors like deleting emails when I accept a calendar invitation.
I've used K-9 on Android and while I liked it, I choose to not receive email on my phone. I do recommend it to non-technical family and friends, and it "Just Works" for them.
The idea that K-9 is "transforming into Thunderbird" sounds utterly fine and dandy to me.
How about we count our blessings and celebrate that FOSS software continues to give us options. Remember how the future of the project was looking uncertain a few years ago? Free Software is a community effort, consider contributing to the effort [0]!