Except all Linux distros rely on the Linux kernel and repackage a vast number of GNU utilities which are maintained by... hundreds and hundreds of engineers.

If GNU/Linux were to die, so would all the distros, because it’s far too much work for any individual distro to maintain all that codebase. In much the same way, the intensive bit of maintenance work with Firefox is the rendering engine; there exist lots of “distros” (forks) of Firefox but they all rely on the same underlying codebase. It’s just not viable to have more than a small handful of rendering engines, much in the same way that there are only a very small handful of operating systems that can run on modern hardware.