You mean Metro/Modern? I never see it anymore, and to be honest I prefer the 10 look over Aero, despite being a Millennial vaporwave fan. I haven't spun up 7 in years and years, and I don't miss it at all. But OS X 10.4's Aqua was the peak.

But also, why wouldn't UI changes be possible if the source was open? I remember WindowBlinds and patched uxTheme.dll in the XP days, and that was /without/ source being available. So in this hypothetical, what's stopping hackers from backporting the things they like about 7 to 10 or adding more rounded translucency?