WSL is vastly different from the old-school NT subsystems. Both in the technology and in the result. And rightly so, because while SFU&co was in some way more integrated, it effectively was a separate platform from anything else, and who want to support that? Arguably the OS/2 subsystem did not have this problem because the environment/ecosystem to support was way smaller, and it was making existing binaries made for this system work. So on that last point, yes WSL is similar to the OS/2 subsystem; but it could not have been like that in SFU because Linux was not seen as a serious competitor at the time (and well, it actually was not...); and the price to pay now that co-evolution did not happen is a more segregated environment.