He's 25 and this is his SIXTH season in the Professional peloton. His results have been getting worse with every season, though he did add a few more heavyweight results in 2013 and 2014 - Gent-Wevelgem and E3. Early developers don't always go on to have stellar careers - this is especially true of say some football players who stand out in the youth / U19 / U21 ranks internationally yet only go on to receive a handful of caps at the top level as they stop improving or have reached a physical peak at a young age and everyone else catches up - if Sagan had rode the U23s until the end of 2013 then we could be talking about a rider with a very limited pro peloton palmares. As it was he turned pro young, got some results quickly and maintained that for a year or two. But it's downhill since 2012 in terms of numbers of wins.
I partially agree with you. It's true Sagan looked like a beast during the his early career. But I don't think we have seen peak from him. Hi's getting better curve was just soo, soooo steep. I actually think he's getting better, just look at times on Kwaremont and Patterberg this year and 2013, he beat the 2013 times. I know the race was not so hard as in 2013, so he saved some extra power, but still. He is not getting worse, only the competion has bridge the gap onto him.
The reason why he stood out so much 2, 3 years back is because there was nobody that could match him in what he's good at. The older cyclists weren't at the level, and he kept beating the likes of Degenkolb, Kristoff as it looked like there was no competition. Since then, his getting better curve has stop rising(hopefully just for a while) and the other young guns, although 2 - 3 years older have caught up to him. And with the age, there comes also the wiseness to play their cards correctly, which for sagan seems to be coming, but have not come yet. Had he had the wiseness 3 years back, he would have had some monuments in his palmares, because he was so dominant at that times. Back than, he had multiple wins because he was so much above the others, but the skill is not enough to win the big ones, as Boonen said, you have to know how to win Ronde and Roubaix, the power itself is not enough.
For the races and also for sagan himself it's good that he has the competition, because without it, after cancellara retires, he would take one cobbled classic after another without having to get better. He definately has the body that can interesting things, he just needs to start getting better every year, both physically and mentally. I think that the stagnation is definately caused by his very easy wins because of two reasons. Everybody started to mark him, so without the experience, he lost too much races in his head, and maybe he also thought that he would be wining like he used forever and ever.
Btw, yesterday, had he had no problems with bike, he would be there with degenkolb, he looked definately better than him on cobbles. If he could challenge him in the sprint, that we will hopefully get to know next year. Or maybe not