What happened in the 90's would imply that the potential market for currently non-Apple users who want to run MacOS on non-Apple hardware is smaller than the pool of current Apple users who would switch to other hardware if provided an easy route. That means loss of market share in their own market.

I'd wager to say that it's probably not much different now.

Apple learned it's lesson in the 90's when it licensed MacOS. While the hope was that the licensees would expand MacOS market share, it instead only whittled away at Apple's own market share. I was an example myself - I have a PowerComputing system lying around somewhere - and it was a sale that would have gone to Apple were they not in existence.

Additionally, as long as Jobs is at the helm, this will never happen. He's made it very clear that Apple doesn't sell hardware or software, but rather the full experience provided by very good integration between the two.