Sastre didn't fall off the cliff immediately after winning the Tour though, he podiumed the next two GTs he entered (including two mountaintop stage wins in the 2009 Giro - albeit the podium came after di Luca's DQ).
And also, I didn't ever say Evans didn't deserve his Tour win. Just that it's false to say that him winning proves that there is no problem with a higher BMI when it comes to winning GTs and *therefore* thinner riders should bulk up as it won't be to their detriment as Evans proves you can do it. Because there are a lot of factors involved in winning a GT, and your BMI is only one of them. And different riders with different VO2Maxes, different aerobic and anaerobic capacities, different training regimes and different season's goals will have different physical shape and characteristics, and your actual physical body size/shape is only one of the many, many factors - some of which you can and some of which you can't control - that go into winning a GT. One of those is pure luck, but every single GT win in the history of cycling has a luck factor involved in it. Sure, some more than others, but ultimately as the cliché goes, to finish first, first you have to finish. Cadel Evans deserved to win the 2011 Tour de France because he completed the course faster than anybody else. Schleck didn't deserve to win the 2011 Tour de France because he raced like an utter coward in the Pyrenées and waited until too late in the game to take the risks to overcome his shortcomings in the TT against the more balanced GT rider, Evans.
If Sagan stays within a country mile of the elite climbers in the field on the Rettenbachferner, the apocalypse is nigh. Even in a limited field where the only ones who haven't already lost fairly significant time are Rui Costa, Jarlinson Pantano, Wilco Kelderman, Geraint Thomas, Simon Spilak, Ion Izagirre, Mathias Fränk, Tejay van Garderen, Andrew Talansky, Sam Oomen, Tiago Machado, Michele Scarponi, Miguel Ángel López, Joe Dombrowski and Przemysław Niemiec - with three straight mountaintop finishes, if not one of them can drop Peter Sagan, that's going to blow people's minds.