With the exception that the Windows Subsystem for Linux can just rely on the opensource glibc et al. So, they basically 'only'[1] have to implement the syscall interface of the Linux kernel. They do not have to do any work to reimplement all the core Linux libraries in a compatible way.
Wine has a much harder task. Since the common Windows libraries are closed source and non-redistributable, the Wine project basically has to reimplement every API/ABI that Windows provides. This is a much larger and harder project.
It would be a nice gesture of Microsoft if they released some of the core Windows libraries as open source. But of course, that's not going to happen.
A few years ago, I'd have agreed with you. I used to be a card-carrying Microsoft hater for many years. And I still agree that the chances of that hapening are small, but given Microsofts changes over the last few years and the stream of open source releases, who knows any more...
Oh, I think it is extremely likely that, assuming no major leadership changes, Microsoft will open parts of Windows. But I think that they'd follow the macOS model - open the kernel and parts of the low-level user land. Enough to make small embedded systems in Windows, make developers happy with low-level profiling capabilities, and receiving more external contributions.
I don't think they will open source things Windows in a constellation where people could start making Windows distributions (which could lead to large incompatibilities) or run GUI apps on Linux or Windows.
everything they have open sourced has been developer oriented, nothing really in the realm of users applications and gaming. So I am doubtful, to say the least, that they will help wine in any shape or form in the foreseeable future.
Yes, the covered surface is much larger. Frankly I don't get why MS insists on keeping Windows closed source. I get why you'd want to keep something others might be able to easily reuse closed source, but I don't get why Windows developers don't get to see Windows source code. In fact, having worked with Linux extensively for the past decade, I don't think I'd be comfortable doing serious work on a system where I don't have source code for debugging and documentation.
