The Carl Pavanos, Kei Igawas and Jaret Wrights were horrible signings but they were not the reason why the Yankees didn't win this year.
As for Santana, it doesn't work like that. You can't simply say that if he was with the Yankees, they would have made it to the playoffs. There are a lot of intangibles that you can't just plug in his numbers and take out the numbers of the players he would have been traded for. The Yankees may have been better or they may have been worse.
The Mets signed Santana and they fared no better this year than they did without him. If everything was reversed and he went to the Yankees instead and the Mets did exactly what they just did this year, you would have said the same thing: if the Mets had traded for Santana, they would not have collapsed.
Do you see how you cannot project outcomes simply by making assumptions based on what they did under different conditions?