It's true that Stannard has to pull, but in the last 1,5 km it's Sagan responsability, and he was on Stannard's wheel. Sagan was the main favourite, he was "obligued" to win, because a 2nd position when you're the favourite it's nothing. And Stannard didn't see this as well as Ciolek or Cancellara.
I'm not sure what "obligation" has to do with it at that point. Sagan didn't need the break to stay away, although if they get caught then he has to contend with Cavendish or a counter-attack. Still, if I'm Sagan I'll take those chances over pulling the break 1.2 km (not 1.5). On the other hand, Sky had no cards left at that point: if the break got caught, Stannard gets 20th. So Stannard thinks "if I don't pull, then who will? Sagan? He doesn't need this. Cancellara? He's not going to pull Sagan to the line. Maybe he attacks. If he gets away, then we're back to the same question, but now it's for second. Better to keep the pace high and seal top-6."
It would have been interesting had the break paused and Phinney caught them. Would Phinney have a chance? He's a competitive sprinter even in pack sprints. But he would have been wasted from the long chase. So probably not.
Also, there seemed the undertone of question in the post-race stories that perhaps Phinney went against plan in chasing. He was supposed to support Gilbert and Hoshovd, it seems, not go after his own result. But in the end he had a brief window of opportunity and went for it, and Gilbert and Thor weren't in position to take advantage of that.
Funny how Trek finally gets it's MSR after years of so-close with Cancellera with it's sponsorship of a South African team. The Madone-7 does well in the wind tunnel, better than the Cannondale Evo. Aero-tuned bikes have done well here.Worth rewatching: video here.