Some parts have already been open-sourced. For example, the console host component has been open sourced here: https://github.com/microsoft/terminal

And the FAT filesystem driver: https://github.com/microsoft/Windows-driver-samples/tree/mai...

And also PowerShell: https://github.com/PowerShell/PowerShell

And most of .NET: https://github.com/dotnet

Maybe they could consider an Apple-style approach: open source the core of the kernel and text-mode user space but leave the GUI closed.

Of course, open sourcing everything would be even better, but that might too big of a step for them. Open sourcing the non-GUI core could be a good initial step, whether or not it ends up going further.