It is less essential these days than it used to be. There are replacements for just about everything from other projects. Many are under even more permissive licensing. A userland is not enough to form an OS either. It is enough to be an organ donor for other operating systems. Each Linux distribution could be considered its own OS.

Also, much of the development behind GNU components were meant for other operating systems than the GNU operating system. There would be many more working on the GNU kernel if that were not the case.