On the other hand, I can't imagine leaving an app for that reason.
As one who fairly loathes Material Design (especially on mobile), I was hoping cketti would fix what's wrong with Thunderbird, rather than Thunderbird ruining the best Android email client out there, which is now looking like an almost certainty.
I'm neither for or against Material Design, but when I used Android, but MD was generally a good sign that the app was well supported, and the devs cared about UX.
When I'd search for an app, the high quality apps were almost always MD. Maybe k9 was an exception, but I'm sure they lost potential users.
Design for me is one of main important on mobile platforms. If the design is bad, is basically a big no for me at least. I wouldn't be leaving because I wouldn't be installing in first place.
I was using Outlook on Android, but I was finding it pretty heavy/slow. Looked for other alternatives, like K9 and Spark, and choose Spark exactly because of the UI/UX.
I get the feeling of wanting a thing you're used to keeping the same looks, but k9 definitely does not have a good user experience. I don't know if the new look will be that, but definitely needs a face lift.
As one who fairly loathes Material Design (especially on mobile), I was hoping cketti would fix what's wrong with Thunderbird, rather than Thunderbird ruining the best Android email client out there, which is now looking like an almost certainty.
I really hope there's a good fork.