That's a STRAW MAN if I have ever seen one. You are analogizing Seles being stabbed to a team losing the Super Bowl. In the latter case, you would be arguing: "What if David Tyree didn't catch that pass from Eli Manning?" or "What if Scott Norwood didn't miss that field goal?" That is in no way analogous to, "What if Monica Seles were not stabbed by a crazed Steffi Graf fan?"
In fact, do you know the best analogy? The 2004 Olympic Marathon. Vanderlei de Lima was leading the Olympic Marathon at the 2004 Olympics when Cornelius Horan, a defrocked Irish priest, tackled Lima, delaying and semi-injuring him. Lima finished 3rd. There were PLENTY of people in the press and on the message boards who called for awarding Lima a gold medal on the ground that he would have won if he were not tackled.
It only appears like a Straw Man's argument because you have overvalued the importance of the stabbing. In effect, the stabbing is no different than an injury which takes a competitor out of the game for a period of time. Only when you add the melodrama that seems to surround women's tennis, because of the loser mentality of most of its fans, does it become the singular event it is in your mind.
In other sports, players sustain injuries that end or curtail their careers. When that happens, fans of that player and the team just move on. It makes no sense to engage in "what if" scenarios because the conversation in that sport regards such thinking as ridiculous. Where are the Sterling Sharpe fans who claim that Jerry Rice's records should all have asterisks next to them? Where are the Bo Jackson fans who claim that Emmitt Smith's records should all have asterisks next to them? In pro football, injuries happen; players with great potential are murdered, die, or end up on bad teams with bad coaching. It is all the same. No one stops to cry or whine about these things. If a player wants to be great, regardless of his social or physical circumstances, he must overcome. See Dez Bryant or Adrian Peterson. Crying about things doesn't make you great.
By comparison, Seles never really came back from her injury and her fans still want her to be the greatest ever. It just doesn't work that way in other sports. But it works that way in women's tennis because of the loser mentality of some of its fans. If your analogy proves anything, it is that that loser mentality can from time to time contaminate other sports. So much for your analogy.