Open source Windows is an interesting premise, but Windows-focused Linux is already a thing going on. Not only has Microsoft basically adopted Ubuntu, but most of their recent projects have been open source. They are actually one of the most numerous contributors to the Linux kernel and it's mostly to make Ubuntu run better on Azure hardware and to make Windows Subsystem for Linux more effective.

I wish Microsoft would just release an official compatibility layer. Let us run Windows software in an officially supported sandbox with directx and all. Hell I'd even pay for a license.

Windows Subsystem for Linux Is what it looks like right now.

But if they were to fully dive in on building a distro, it would just be a first step on the EEE pipeline.

The better question is why would we want that?

Microsoft has historically not been friendly to anyone else. Until they prove otherwise, this is going to be my assumption. It's some form of embrace/extend/extinguish.

I love that Linux is everything that Microsoft is not. I love that I have full control of my hardware. I have control over processes. I have control over packages. And user control is the default.

I can already join a Linux PC to a domain and run VPN. I can easily transfer files. I'm good.

My guess, based on Proprietary Codebases I Have Seen and the apparent general philosophy of Windows development, is that people would react to the now-open-sourced Windows code with either hilarity or horror (or both at once). There would be critical articles in the tech press. Then a small select group would mine it for low-level interoperability, but none of the code itself would be retained. Emulation layers such as WINE would end up being able to reproduce Windows' quirks more thoroughly, but finding the important bits in a mess of Someone Else's Code would slow down development as much as having an exemplar would speed it up. This all assumes that the code was released with an acceptable license.

On the Linux side, mostly a wash beyond some small interoperability gains, in other words. What would happen on the Windows side, I wouldn't venture a guess on.

Teams would possibly suck less, oh dear God how I hate teams with a passion, teams in Linux just fucking sucks.

Would WINE and Proton's development rapidly accelerate?

Maybe, it would also depend on the license used. "opensource" doesn't necessarily mean GPL, so even if MS opensourced Windows, the license could prohibit the code from being used in Wine or other L/GPL'd projects.

And yes, Wine would still be necessary for people who use Linux mainly not because it's "free", but because of its architecture and ecosystem, and would like to retain compatibility with Windows applications.

Would Windows tools like their file explorer become dominant

Unlikely. File Explorer sucks, and so do most of tools built into Windows. Which is one of the reason why there are so many third-party utilities in the Windows world (like xplorer2, Notepad++, Paint .NET etc). The Task Manager is decent now I guess, but we already have a few clones that does the job. PowerShell is also decent, but it's already opensourced and cross-platform. Can't really think of any other built-in Windows tool that's better than Linux alternatives.

Would things like NTFS be installed by default in most distros?

It already is. The Linux kernel now includes the NTFS3 driver (developed by Paragon) and it does a pretty good job. exFAT also no longer has legal issues since 2019 and is also part of the kernel.

Do you think a lot of people and businesses would jump on a chimera distro that was half and half?

Probably not. The biggest roadblock to Linux getting mass adoption is the lack of a big hardware backer. Most people don't mess around with operating systems, they just use whatever OS is preloaded on their device, and it has very little to do with compatibility. Look how gimped Chromebooks are, yet people still buy them, and that's because you could just walk into a Best Buy or whatever and pick it up from the shelves. You can't do that with mainstream Linux distros. Companies like System76 are on the right track, but they're still far, far away from mass-market reach. It would take someone big like Google to package Linux nicely into shiny hardware, partner up with retailers and hardware makers, and market the hell out of it.

It would be interesting. There are a number of bugs that are depended on so are preserved across updates.

Microsoft is also/was terrible at sharing documentation.

I think Windows would have a huge resurgence. The chimera would be used, but a large number of Windows distros would be created.

OTOH, I am terrible at predictions.

Only thing it would change is if we could run Windows apps more cleanly. Otherwise who cares. Most devices out there are already POSIX like systems anyway. Linux, BSD, OSx, Android. It is Windows that is not compatible.

It would be a signal that MS is throwing in the towel as far as Windows as a separate OS. However, I suspect they already have some kind of Windows compatibility layer of their own that they've been working on. It would probably have some sort of limited source availability, as opposed to a full open source FOSS licence. If that is the case, it could actually undermine WINE. If MS could save face somehow and fully embrace Linux, they might do it, but I doubt it will happen. Who knows.