I assume Microsoft couldn't push their own solution because of the anti-trust restrictions. Now that those have expired, perhaps they could unless they're afraid they'll get lawsuits against them again.
Is the PC & OS market still as MS-centric as it was back then? I know they still own the majority of the desktop, but with OS X out there, and more fragmentation to mobile, it feels like it would be much much harder to build an anti-trust case against Microsoft.
Globally, yes. I think parent was talking about the US market where OS X is slightly more prominent and according to StatatsCounter surpasses the market share of Windows XP and Windows 10 combined. Microsoft still controls 3/4 of the desktop market according to these stats, but that would probably not qualify as monopoly anymore.