I partly agree with you, but there are some additions. Yes, Sagan was in bad position at patter. Why, because he was getting back from his second bike change, that happened before the decisive crash. I saw a photo somewhere, from which I could see, that sagan may have been the last that was not affected by that crash. Then, he made quite effort to get to front on paterberg, which was the bet possible place to do it for him. Why, because it's steep and due to sagans position on bike, he's much more comfortable on cobbles when the course is very steep. On something less steeper, or even flat(kwaremont is not very steep, only long) he has more difficulty to stay with guys that sit more in the back. And as result he wasn't feeling that well by the time they reached kwaremont, as he stated in post race interviews. He even said he didn't think he could win at that time. So sagan's performance on kwaremont wasn't what he's capable of.
To sum it up, we need to wait for kwaremont and paterberg duo in reversed order in ronde to really see, what shape is cancellara and what shape is sagan, and others. Cancellara is certainly better, but it probably is not enough. I personally think, that cancellara will not be able to drop sagan on kwaremont(he couldn't do it last year too, and I think fabian was in better shape then), and it will depend on amount of strengths that sagan will have saved to paterberg, if he can hang on. For sagan actually, I might be better if isolated with fabian after kwaremont like last year. As a result, fabian might refuse to work either and terpstra and stybar and maybe someone else will catch up. However, this doesn't really need to worry sagan and he can save the energy for pater, where fireworks will happen again.