I have been expecting this for a few years now, why? I will tell you.
Microsoft development [those who code windows] have been using their own Linux build for some time to code new windows versions. When i got my current machine, it came with a fresh W10 preinstall, so i thought i would try it, but It had installation problems. Not having used Windows for many years, I joined the MS forums for help [fatal mistake] I filled in the introduce yourself section of the form to join, stating I had not used windows for years but using solely Linux, anyway the result was I never got any usable advice on how to fix the audio problems, so in a huff, in my last post basically said they were unless, and I was going back to Linux. About 3 weeks later, I got a PM from a different department at Microsoft, asking me to become a Mentor [I thought strange, after my last post] well I declined the offer,[shortly after I joined this forum] and a couple of months later WSL was released.
So the signs have been there.
edit.. The worse probeable outcome, if the next generation of Windows it Linux based, it will then encourage the idiots of this world to develop more nasties aimed at Linux systems