> And since I sense the question coming, Android, OS X/iOS, ChromeOS might use an UNIXy kernel, but its exposure surface to applications is so small, that it can be easily exchange for something else, it is just a matter of cost.
Android, macOS, and iOS expose almost the full set of POSIX to the programmer. Now, you may argue that it goes mostly unused, but it is there, and there are people using it.
As for macOS, and iOS, if Apple removed POSIX how many apps written in Objective-C and Swift (not UNIX cli tools ported from Linux) would actually be affected?
Very few, because those written in C and C++ ported from other platforms are most likely making use of ANSI C and C++ standards.
Android, macOS, and iOS expose almost the full set of POSIX to the programmer. Now, you may argue that it goes mostly unused, but it is there, and there are people using it.