If the ecosystem doesn't care about POSIX it can hardly complain about choices made by a library whose purpose is to implement the POSIX standard. It also has

If the ecosystem doesn't care about POSIX it can hardly complain about choices made by a library whose purpose is to implement the POSIX standard. It also has nothing to do with ABIs - the idea of a standard is to provide a common subset of functionality supported by all platforms that it targets. Nothing about musl or any other libc implementation prevents developers from including the linux headers and using whatever linux-specific functionality they like.