Not sure if this is what you're trying to say but it
sounds like this is just a thing that designers will hop on a new trend every few years or so.
Except that I remember a few changes in design trends and many of them were for the better, others were a matter of taste that didn't really matter.
But "Flat UI" was a step down in almost every aspect (or whatever name Google called pretty much the same thing, and so did Microsoft have a name for their particular flavour of low-poly coloured squares). It doesn't even look good (personal opinion). Indeed it was confusing, after a while we got used to it, so it didn't hurt usability quite as much, but it was never good.
Just an excuse to not learn about actual usability guidelines. And with that, excuses to hide your anti-patterns under, which the tech giants all happily do. Tiny links function sometimes as buttons but all buttons are coloured squares so basically any layout element could do something but usually not, and with a touch-interface this adds even more stress because you can never tell where you can comfortably rest your finger to scroll or zoom, if even possible, which is not indicated because the cute flat scrollbars tend to fade away if they're not in use etc etc. Nonsense!
If most of the normal interface is already uncertain in functionality, then users (or consumers, eyes, or products) will not complain as much when they make a certain action harder to perform on purpose.
We got suckered into hostile UX. So maybe we can stop doing that now.