> If Stallman had started with a kernel, there would be very few people who had the legal right to run any utilities or apps on the new kernel

That's true in isolation, but is missing vital context. In the 1980s a user of proprietary Unix usually had no way to make the ls utility or a C compiler or a text editor run on an new Unix-like kernel. And even if he had the technical means to do it, he probably did not have the legal right, i.e., it would have been a violation of copyright law.

In the Unix world, there was no tradition of binary compatibility: you couldn't just take a binary compiled for NeXTSTEP and run it on Solaris or on Linux: you needed to recompile, which means you needed a C compiler and the source code for the program you want to run, and most users of Unix in the 1980s didn't have a practical way to get the source code (and if he did have access to the source code, using to to port the program to a new kernel was probably a copyright violation). Stallman could have tried to start a tradition of binary compatibility in the Unix world. That is one of the strategies he could have chosen for the GNU project: i.e., he could have picked a proprietary Unix, Solaris, say, and make sure the kernel of his GNU system could run all (or most) binaries that run on Solaris. But that strategy ran the risk that the owner of Solaris might have sued to stop this and the courts might have sided with owner, requiring the GNU project to shut down, pay the owner (Sun) a lot of money or start over with some other strategy.