Break it down, slam by slam.
2003 AO, Agassi beat Schuttler in the final. Fresh from his devastating blow to Agassi only 4 months earlier, Sampras would have done the same in Melbourne.
2003 FO, Ferrero beat Verkerk in the final. When Sampras was determined to do so, he won on clay, unlike Djokovic who's still trying for that elusive FO. Sampras not only won on clay, but he went to Russia and was the catalyst for the USA winning the DC on clay, which was huge back then, unlike in today's plastic tour. Knowing it was his last FO, Sampras would have pulled out all stops to win on clay again.
2013 W, Federer beat Philipwho? in the final. Quite simply, Sampras does NOT LOSE Wimbledon finals.
2013 USO, won by Roddick. Enough said.
No one would have stopped him. This is not fanboyism and not a troll. Like I said, break it down.
That is all.