Just to hammer this point home some more: There are actually also significant disadvantages to the Kernel being closed source.
As an example: Windows has had an official port of OpenDTrace for a year now. But sadly, the documentation for it is almost nonexistent, and just finding out which functions you might want to instrument (and what context data is available there) is much more painful than on other systems.
OTOH, if you're on OpenSolaris or Linux (where you'll likely use bpftrace instead), documentation on the matter is abundant.
Backwards compatibility is a big one, and arguably the key principle behind all of Windows' evolution. Yes, WINE, yes, apps all moving to the browser, but there's a long tail of applications that won't work as well under WINE as they do on modern Windows versions.
Like Apple did before them, they will take an open-source kernel loving produced by thousands of willing programmers (BSD for Apple, Linux for Microsoft) and put their own proprietary GUI (Aqua for Apple, Windows for Microsoft) on top of that.
Fundamentally, what advantage is Windows going to maintain with a closed source kernel? How many customers care that MS isn't using the Linux kernel? Not many. The bean counters are eventually going to start ask questions, like "why are we paying to maintain our own kernel when we could just use Linux?".
I don't see what strategic edge MS is going to hold maintaining a parallel OS long term. If they change rapidly they lose the legacy advantage, if they don't OSS will catch up. It isn't like NTFS is giving them a huge advantage over ext4. The key patents will all be starting to expire too.
The only effective resistance to OSS seems to have been this service-based cloud model that exploded onto the scene in the last, what, 10-15 years?