> You can't really optimize, in the A/B-testing sense, an entire design.

That's just crap. You can test anything, including entirely different versions of an app. The idea that you can't test good design is what designers tell themselves to justify their operating on gut feeling and pulling opinions out of their asses.

There isn't anything a user does that you can't come up with a way to test, designers are just afraid of actual data showing them they're full of shit. They don't want their ideas challenged democratically by users, they want their own ideas used instead because of some misplaced sense of taste. Engineers do it right, test, iterate, improve, follow the data wherever it leads regardless of your own ego.