Unfortunately for the real world, the refs missed calling him down and lord only knows why USC didn't challenge the play.
To this I say: Texas did their job. If there's ever a questionable call that you think might go against you, your job is to line up and start the next play as quickly as humanly possible, because once that next play starts, the refs can't go back and review a previous play.

As for why USC didn't challenge the play...It's because they can't. No such thing as a "challenge" in the college game like there is in the pros. They could have called time out...But they didn't.

Now, on to the meat of the point here...The effect of an overturned touchdown would have been nil, because Texas would have scored on the next play or two anyway, and there would have been little difference. Except, perhaps, that USC might not have scored their field goal at the end of the first half, thereby making the final score 41-35 instead of 41-38.