We know. The culture in Unix was always sharing source and user collaboration. When AT&T finally tried to exert control there was a large movement within the community to go free of AT&T code. GNU and BSD were both a reaction to that.

The rise of x86 and the availability of free Unix (in the form of BSD or GNU) was destined to destroy the proprietary Unix market. In 1991 neither BSD nor GNU had kernels booting on x86, leading Linus to eventually release Linux in August.

Linux had the luxury of being the only readily available Unix-like kernel on x86 for several years.

The USL v BSDi lawsuit slowed BSD efforts for nearly two years while Linux gained mindshare and features (specifically x86 features & drivers). Once the suit was settled out of court development took a long time to regain momentum.

Net/2 was released in June of 1991 (before Linux!), so if the USL lawsuit hadn't happened, we might all be running that instead.