Bradley Wiggins can't ride on cobbles, neither can Tony Martin or David Millar. Roubaix isn't about time trial skills, but bike handling on cobbles, positioning and a lot of endurance.
Boonen can win races solo, in a small group sprint and in a bunch sprint. Cancellara can only win solo.
Cancellara only rode one year in the U23 ranks(and even then only a few races), so that argument makes no sense. He was 23 years old when he got fourth in Roubaix. He rode his first Ronde van Vlaanderen at age 22.
Boonen got second once in LBL for espoirs back in 2001.
Cancellara has also never won a mountain stage race in his life. Tirreno-Adriatico 2008 wasn't even Protour that year and only had one tough Muro stage. Tour de Suisse was tailor made for him. Tony Martin was second that year and he's no climber either(but better than Cancellara anyway).
Also Strade Bianche is just a 1.1 race, something a lot of people tend to forget. I don't even think Boonen wants to win that race lol. Long distance asphalt classics? Which one would that be? Milan-San Remo? As if Boonen did not have the capabilities to win that race. He finished on the podium twice already. They ride the same classics every year you know. With the exception of the Amstel Gold Race.