They would only be obliged to open source any extra code they added to the kernel. If whatever they add lives in user space then it can be closed source (that’s one of the key differences between GPL 2 and 3 and why Linus refuses to use GPL 3). That said the problem with Windows at this point isn’t really the kernel, it’s all the user space crap they built on top of it.
A man can wish but they would never do that because of GPL and thus having to also open source anything built-in/in-top by them (afaik?)
Not really. Android and the google layer on top is a pretty good example of what you can do.
They would only be obliged to open source any extra code they added to the kernel. If whatever they add lives in user space then it can be closed source (that’s one of the key differences between GPL 2 and 3 and why Linus refuses to use GPL 3). That said the problem with Windows at this point isn’t really the kernel, it’s all the user space crap they built on top of it.
Even then, they can just have an open source shim and a binary blob for the driver, a la Nvidia.