Really? Unix with xwindows wouldn’t run in the 90’s either? I think there are a ton of Linux heads that will dispute that.
If the question is whether Jobs would have done OS X in the nineties if he had been in charge of Apple over that decade, the answer is yes. Jobs proved that when he brought out the NeXT computer, which was a more immediate predecessor to OS X than the old Mac itself was.If it had been as easy as you make it sound, Jobs would have used Unix for the initial Mac - but that was a bridge too far at that time. The hardware it required was still too bleeding edge in 1988, which was why NeXT failed.
You could say that Linux has been the geek's NeXT.
When he returned to Apple, Jobs brought out OS X and delivered the whole commercial package - finished, well-integrated hardware and software, and marketing/advertising, sales, and support. Completing the Apple Store network is still a work in progress because you can live in a major population center without being close to one of the stores. Seems like that would be a good place to put some of that cash hoard Apple is known for . . .