Windows has a giant attack surface because of the sheer indefinitely-legacy-compatible, multiverse-sized codebases of unsafe, unprovable, low-level code in C/C#(CLR)/asm.
The last beta copy of Windows code I reviewed looked like it was maintained by thousands of contract programmers and interns on artificial deadlines whom can't spare a second to clean up nasty technical debt.
With Russinovich and Azure's leader promoted now, I hope they're gonna nut up to take a stab at incurring temporary customer wrath by remaking Windows from (mostly) scratch with less duct tape and more lessons learned. It could even go full open source (for core OS, Visual Studio and shared libs) to dig at Apple (whom is already semi-open source). Given their resources, they could do a modern, nearly ground-up, respectable-as-OpenBSD OS in a combination of functional/multiparadigm languages and not fail too terribly at it if it's previewed/seeks feedback early-and-often. Maintaining marketshare is about taking-on hard-but-necessary choices for long-term gain, not seemingly safe moves of tacking on some tweaks and security add-ons to the overall status quo.