I think A/B testing may have value in selecting a specific path preferred by 85% of users, but it seems like alienating the remaining 15% doesn't feel great.

95% of users might not want the double-textbox, but 5% might, and some might want to use it someday.

I think, make enough of these decisions, and there's bound to be mounting friction for users with different preferences navigating your app.

Ideally, the app should offer a way for the user to intuitively configure the app to their own liking.

When an experiment passes, you don’t really lose that 15%. Most of the time they just get used to the other behaviour. Case in point - we’ve been whining about Google’s deterioration of search queries for ages, yet we’re still using it.

Ideally, yeah maybe, but why bother with extra implementation, support, costs and etc., when people fold and use the new way anyways?