The ongoing trend of “flat UI” is largely not due to processing power though. Even inexpensive computers have CPUs and GPUs that could push very fancy graphics without problems, see what the same machines can do in game graphics (and I don’t mean high-end gaming, I mean the kind of simple gaming that can run on a low-end laptop these days). Some of the early GUIs in the 1980s had “flat design” due to performance limitations, but that went away in the 1990s. Today it could still be a reason in some embedded system scenarios with simple microcontrollers, but not in a desktop or laptop computer, and also not in smartphones or tablets.
The reason we have the bland flat design is the same why we still have things like “all surfaces are ugly glossy black plastic” (luckily this one is on its way out) or “war on physical buttons” aka “touchscreens everywhere”… it’s simply a design trend.
Not even buying things on blu-ray is safe anymore from being tainted with “AI” nightmare fuel.