It was not an extremely functional product. It was largely there to get government contracts where one of the requirements was POSIX conformance, even if they weren't using it.
Imagining they had first-class support for it (a major undertaking, I'd guess, but anyway) how many people would use it? I'd guess it'd be about the same people who use Cygwin now.
Why would you guess that? Because cygwin is nothing close to first class support. It has a lot of friction associated with its use. You're better off just running linux in a vm if you want posix on windows, and trust me plenty of people do that these days.
Yes, but I think Microsoft's ultimate concern is how many people will want to use such applications. How many will? Think about how few people run, say, PHP applications under Windows. Even though it's possible. (Well, actually, often it won't work right because PHP developers don't bother to test with anything besides Linux)