The fear is unification of iOS and macOS, which Apple has hinted at occasionally. macOS doesn't need to disappear completely for many fears to be realized. The more application APIs are unified, for example, the more pressure there becomes for macOS to deprecate and let stagnate its POSIX interfaces. And you don't need the POSIX interfaces to disappear completely for the environment to cease being targetable by most open source projects, especially command-line projects. Windows has a significant number of POSIX APIs stemming from Windows NT 4.0 POSIX compliance, but it's just not a practical Unix target based on that subset alone.