I guess operating systems acquiring HiDPI support is one of the reasons going for the flat look.
My own guess is that someone has a color e-ink display in the works.
The last time I checked, cheap color e-ink displays simply couldn't show enough colors for photos. They were fine for charts and graphs, and fine for color-coding text, but if you tried to do something photorealistic in them, well, it was worse than old 90s-era 16-bit displays.
Flat icons with few colors would work spectacularly well on such displays.
If these icon changes are actually in support of color displays that draw almost no power and are completely readable in full sunlight, then bring on the ugly icons please!