A/B testing does not inherently lead you to good design. If you A/B test what people want based on their revealed user preference, you get gambling apps and doomscrolling.
"I want a steak dinner." Yeah, but based on usage patterns, here's some cigarettes and beer. /s
Even better is actual testing.
They put windows 95 in front of users, asked them to perform tasks, and, at the end of the test, asked them to shut down the computer. Almost everyone clicked on the start menu first, so that's where they put the shut down button.