Are they actually crucial, though? There was a ton of gnashing of teeth about iOS 7 (and other "flat" designs) but one rarely hears those anymore… and it's not at all clear to me that less-savvy users are having more trouble operating their devices than they did a couple of years ago. In fact, it seems just the opposite, that these devices have steadily continued to integrate themselves into people's lives more than ever before. Come to think of it, it seems that the gnashing of teeth has shifted to that (these screens that we spend our lives staring at!) over the last year or two.

I'm sure there have been at least some regressions in some aspects of some user performance measurements, but it seems really clear at this point that it hasn't been a catastrophe… or even a significant problem.

well you get things like Google's Material design that have things like depth perception and strong color contrast.

It's not like we're abandoning everything, it's more about discovering new ways of doing things and then mix and matching until we get to something useful and coherent.