Cancellara had put some time in training for sprint finishes, so that may have influenced his decision. He had maybe 1 or 2 drops left in the tank. Cancellara thought the Sky boys will bring it back together & maybe he had a chance at the sprint. Maybe that was his thinking. Cancellara didn't do his typical multiple attacks, multiple efforts. He was positioned badly most of the race and had to constantly work to move up. At one point, he was a few groups back. Once he came to the front, he had to recuperate from the effort just to catch on.
Boonen did too much, but he was by far the most impressive rider, IMO.
Boonen was cooked at the end, though. If Sky had attacked, Boonen wouldn't have been able to chase.
Sagan was also very smart. I appreciate the go harder before the cobbles, knowing that the better riders will catch him. My strategy on hills

I take off in the flats before, knowing the group will catch me and pass me. Better chance of me latching back on with that head start. It was obvious that Sagan was done, spent, at the end. He didn't even contest the 2nd place sprint.
Wiggins and Cancellara may have time-trialed themselves back, but all were cooked.
Terpstra did it best. He got the OK from the team to attack, then it was balls out! way over threshold, totally gasping, all mental since body was done. Win-win for him. If he got caught, he was helping the team and at least 6 or 7 of the remaining 10 would have been dropped. He still could have latched on to the new group and maybe saved some for the velodrome. Fortunately, no one chased, so he kept on gaining. I got a bit emotional when he did, in the velodrome, all by himself, crowd going crazy.
Degenkolb was the only one that had anything remotely left from the rest and also deserved 2nd place.
One of my favorite Paris-Roubaix. Cobblestone sections=riders dropping like flies. The break kept on losing riders to flats. I kept on cheering for Dayer Quintana, he always seemed to just make it out at the tail end. Same with Kristoff. Both had to abandon, though