It's not really other riders' fault though. It's only sensible tactics when you have somebody who is as strong as Sagan is AND as good a sprinter as he is, to make them tire themselves out in order to be able to then take advantage by gapping him. The problem is, when you have riders as strong as Wellens and Dumoulin up the road and working well, they're going to put time into Sagan working on his own, but by the time Sagan is spent they'll be out of sight. It's no different to everybody hitching a ride on the Boonen train while Cancellara rode into the distance in Roubaix 2010, now, is it? As Óscar Freire said, in order to win, first you must be prepared to risk losing.
The other thing is that an in-form Sagan is a very dangerous, multi-purpose threat. To beat him, you must tire him sufficiently. Just attacking often isn't enough as he is powerful enough that he can bridge a lot of small gaps by himself. When you don't have multiple riders in a group to play the 1-2 trick, you have to gamble on other riders' attacks not being the ones that he can't pull back and wait for him to tire himself out. When everybody in the group isolates him as the biggest threat, everybody will be watching him to chase down that move rather than rider C marking rider D while rider B is expecting rider A to chase. And also, at Gent-Wevelgem Sagan demonstrated that Terpstra using that tactic got under his skin and angered him, in which case he's more likely to get frustrated and defeat himself by doing something tactically boneheaded. He has left the implication earlier in the season that he does feel entitled to be able to contest the win in any race within his remit, such as with his reckless barging of Vantomme after being caught out of position, and though upon review of the specific Tour incident he recanted his opinion of that one crash, Greipel's opining on Sagan after the Tour incident that he felt he had a god-given right to be at the front is telling; he needs to be better at bluffing when it's getting to him if he wants people to collaborate.
In that vein, maybe there are a few riders out there, as per the Quick Step posturing in the spring, who won't ride with him just because they want to see him lose. But there are plenty of riders out there for whom not collaborating with Sagan is actually their ploy to win, because they don't see it as trading a very likely loss (sprinting against Sagan) for a guaranteed loss (the attackers not being pulled back), but instead as trading a very likely loss (unless Sagan keeps the gap bridgable) for a likely loss (escaping or sprinting from a group with an exhausted Sagan). When you think about it, he should be glad they pay him that compliment - he's so dangerous that riders value their chances higher letting the escape go than they do working with him. But the UCI points system holds a lot of blame for that as well, overvaluing some fairly nondescript or anonymous placements whereas if lower end top 10 places weren't worth protecting maybe he'd find more takers when he flicks his elbow. Or maybe not, as he'd still be the best sprinter of the group.