Considering WSL is getting more popular and sees some solid development
these days, what are the odds that this is going to be our sole target
on Windows, some years since?
Microsoft is moving the WSL 'platform' away from translating POSIX calls into Windows-native (W32, Win32, I forget the details), and instead implementing a system where the Linux kernel runs in a custom VM (actually, inside a container running inside a VM, if I understand it correctly). The system then provides the necessary software to bridge the two systems from the user POV. The architecture is supposedly very close to what Google (the software part, not the Advertising company part) is doing with ChromeOS and their Crostini project, where the user's choice of distribution runs (kernel and all) alongside the Linux kernel that underlies ChromeOS, and software inside each separate OS is used to bridge (for example) X11 into the Wayland-based display engine, share files between the two, etc. Supposedly, this was the way they found to solve the performance problems with the impedance mismatch between Windows and Linux design issues (especially filesystem operations).
If this works, then it'll make Windows support easier, since it will be very close to GNU/Linux support.
Hope that helps,