​FWIW:  I disagree​.  For details look at my discussion of  rewriting Linux in RUST  on quora.   But a quick point is this .... Linux original took off (and was successful) not because of GPL, but in spite of it and later the GPL would help it.  But it was not the GPL per say that made Linux vs BSD vs SunOS et al.

What made Linux happen was the BSDi/UCB vs AT&T case.      At the time, a lot of hackers (myself included) thought the case was about copyright.   It was not, it was about trade secret and the ideas around UNIX.   i.e. folks like, we "mentally contaminated" with the AT&T Intellectual Property.

When the case came, folks like me that were running 386BSD which would later begat FreeBSD et al, got scared.   At that time, *BSD (and SunOS) were much farther along in the development and stability.   But .... may of us hought Linux would insulate us from losing UNIX on cheap HW because their was not AT&T copyrighted code in it.    Sadly, the truth is that if AT&T had won the case, all UNIX-like systems would have had to be removed from the market in the USA and EU [NATO-allies for sure].

That said, the fact the *BSD and Linux were in the wild, would have made it hard to enforce and at a "Free" (as in beer) price it may have been hard to make it stick.    But that it was a misunderstanding of legal thing that made Linux "valuable"  to us, not the implementation.

If SunOS has been available, it would not have been any different.  It would have been thought of based on the AT&T IP, but trade secret and original copyright.

Clem