What’s wrong with flat UIs? Skeuomorphic designs have served their purpose of helping people get used to computers, but now that is no longer necessary.

Why was that? What causes such fads? Why did everyone go along with it?

I bet people had the same concerns with the transition from Windows 3.1's flat UI (only basic shapes were used) to the pseudo-3D effects of Windows 95.

Although in 3.1 it was easy to tell what was interactable, despite being flat. I attribute this to the use of standardized components almost everywhere.

But there are things like consistency which one can check for. And should.