Comment by CuriouslyC

I think this is easily disproven by looking at other areas of functional design besides computer UIs. Architecture is a good example - it's obvious there that while there are some things that are optimizations, a large part of the churn in architecture is style. You might claim the modern trend of having lots of large windows to emphasize natural light is an example of an optimization, but that is only "modern" because the materials and processes to affordably make big pieces of double pane glass are modern. Otherwise, the brutalist monstrosities of the last 30 years are completely a stylistic choice. Ironically flat icons and brutalist architecture share a lot in common.