And I'm far from sure that the decisions good for the long term development of Linux would be done by bigcorps if Linus went missing anyway (especially not Google). Bigcorps have bad habits (with high impedance mismatch in the context of the Linux project), like development behind closed doors, and not only time-to-the-market centered but even for some of them time-to-the-market weighting 95% in their choices. Because of that, bigcorps were the cause of the arm branch mess in the first place. Bigcorps often lead big projects that fails big. And Linux is not lead by bigcorps anyway, it's lead by peoples.
Also, the same argument could be applied to lot of people in leadership positions anyway. Maybe Apple would not be Apple without Steve Jobs, but the CEO does not matter more in lots of big companies than Linus matters for Linux. Maybe it would be very equivalent with somebody else, or maybe not, but in the meantime they are in charge, and when they do a good job, they are the ones who matter.
It's not that Linus stumbled on some secret of technology or was smarter than anyone else doing 'Nix development. It's that he got obstructions out of the way that kept those who could contribute meaningfully from being able to do so and allowed others to utilize the results.
The end result is the innovation and technical superiority (by and large) of Linux over alternatives.
On how Unix initially emerged: there was no copyright of software (this changed in 1976), and AT&T were prohibited under a 1954 anti-trust consent decree from participating in computer equipment and software. So when Ritchie and Thompson created Unix, the company could do little but use it internally, and had no reason to prevent its more widespread distribution. Unix exists specifically because AT&T was prevented from productizing it.