I would have been happy to see Goldberg v Austin at any PPV really.
However, Mania 19 would not have been the best time for the match to take place. As we know, the match with The Rock was Austin's last match in the WWE and he went out on a classic, which is the way it should be. Rocky and Stone Cold always worked so well together, and had an epic rivalry over the years so it made sense for Austin to finish with The Rock finally getting te WM victory over Austin. It brought thei rivalry to a satisfactory conclusion.
Goldberg v Austin was a match everyone wanted to see, but in his last match with the WWE, there would have been no way that Vince would have let his top guy throughout that era lose to WCW's top guy throughout that era. Not a chance. It would be signifying that WCW was better than the WWE and we know Vince refuses to hint at that in any situation, let alone with his top guy in Austin.
Austin needed to lose at WM19 to bow out on his back, but Goldberg would never have been booked to go over had the match taken place, especially as he was on a 12 month deal. Had Goldberg been signed for longer and was going to be THE GUY after that WM, then maybe. But if the situation had been the same, with Bill on a 12 month deal and Austin leaving, then Stone Cold would have been booked to win.