Linux could easily have never existed.
At the time Linus got started in 1991, *BSD was tied up in a legal dispute (which was not resolved until 1994), and Minix was only available under a restrictive license (Minix was relicensed BSD in 2000)
If either of those legal issues had not existed in 1991, Linus probably would never have started Linux [0], and Linux's place in history would have been assumed by something else – most likely some BSD derivative or a descendant of Minix.
(Minix's microkernel architecture, while elegant, has a performance cost. However, Minix could have easily been hacked into a monolithic kernel, which is basically what Apple did to Mach, and Microsoft did with win32k.sys. Or maybe it could have evolved towards a higher performance microkernel architecture, such as is found in L4)
In terms of the "technology tree", Linux occupies the place of "leading openly developed FLOSS operating system". If Linux hadn't occupied that place, some other system would have instead. And there is a chance even that one day Linux might be dethroned. (Unlikely to happen any time soon, but maybe some decades or centuries from now.)
[0] So says the man himself: "If 386BSD had been available when I started on Linux, Linux would probably never had happened." https://gondwanaland.com/meta/history/interview.html