Once upon a time UIs were all flat, then UX experts did some research and discovered that introducing 3D elements increased navigability. This must have been sometime around 1988.
Not surprising that modern research agrees, as meatspace hardware changes terribly slowly.
Agreed. What happened here is certain companies (Apple, Microsoft, etc), pushed the "new, modern, 50% less fat!" flat GUIs so they could either (a) run it on cheaper hardware or (b) make it run faster on underpowered hardware. iOS changed to make it run faster. The abomination that was Metro in Windows 8 was so they could run it on phones barely more powerful than digital watches.
The companies made the flat GUIs, then hired some brain-addled UX "experts" to convince us all that it was actually better not being able to figure out GUIs