I think the whole mobile operating system thing has screwed up GUI design to a certain degree. Microsoft, Ubuntu, and GNOME have both been brave and tried something new, but what they ended up with ended up being highly unpopular on the desktop. And to be honest, I think only Microsoft ended up with something truly good on a touch interface, though I admit to not using Ubuntu or GNOME in those contexts, just being aware that they've not really encouraged an ecosystem for applications to work well in a tablet environment, leaving users with only the main shell being friendly. So the loss of optimization for the desktop lead to no significant gains elsewhere.
The way I'm seeing it, Windows 10 seems to be genuinely exciting, and a decent modern desktop, that also encourages cross interface design. Microsoft has learned from the mistakes it made with Windows 8, kept the good parts, and put together something truly great and modern.
I don't really want to be stuck with Windows though as my primary OS. I'm hoping Ubuntu et al actually learn from it.
This is something you'll never normally hear from me, but perhaps they need a Miguel type figure to take a lead in either GNOME or Ubuntu. At this point, at least to me, it looks like Microsoft is the one with the good ideas about how a UI should work and the relationship of an application to the UI frameworks of the underlying OS. I don't want anyone to clone Windows, but it would be nice to learn from it, at least.
Back in the 1990s, nerds like me put together our own "desktops", running random window managers, app launchers, and file managers (if that) that seemed to go together. I'm feeling like the FOSS "desktop" is heading back to that era, of stuff that doesn't really go together, being shoehorned to fit, with no real philosophy binding the system together.