I agree, but I’ve always wondered if flat UIs had a performance advantage. Like, if your GPU just rendered a bunch of rectangles of a single color instead of having to mask rounded corners or draw borders around things, could the UI run with less resources? Maybe it’s conspiratorial but it helps me sleep at night as a remember not finding buttons during the day…
Considering we had 3D controls in the Windows 3.1 era when some people were still running on 25 Mhz 80386 CPUs and 4 MB of RAM, I'm not too worried about the performance impact.