It is equally valid to say that Stallman's starting to write a C compiler and Unix utilities (in 1984 whereas the Linux project started in late 1991) paved the way to getting an open source version of Unix installed on billions of machines.
Though I'm unsure how differing licenses might have affected this - I suspect that really early in it's development the "copyleft" nature of the GPL Linux didn't make as much of a difference, as from what I remember most commercial uses of Linux didn't come until it had already gained significant momentum.
Copyleft encourages a collaborative relationship between entities because it makes trying to play it close to the chest with IP involve more legal effort (if it's possible at all).
Commercial support for Linux was... Sparse... before the early 2000s.
Agreed, funnily enough GNU tools/compilers also ended up getting installed on a lot of proprietary UNIXes because proprietary UNIX was mostly shit (in user space!). At least most of the ones I had the misfortune to have to work on.
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 whereas GNU's utilities and apps (e.g., Emacs) were immediately useful (i.e., without breaking any copyright law or violating any software license) to a large population, namely, anyone with an account on a proprietary Unix system, which explains why Stallman chose to start with the userland.
> 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
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.
I found a shorter reply to your comment, which I will now write:
>when it comes to the GNU project and the whole Free Software movement is the ability to run _any_ program, be it non-free software or free software.
Just because the GNU project allowed non-free software to run on GNU doesn't mean there existed in the 1980s a userland that would run on GNU if GNU had consisted of just a kernel (or just a kernel and a C library and a C compiler).