First of all, Sagan is much more than just a classic rider. Guy won more than 100 races (which is a very rare feet nowadays), and he's only 30.
Second, if you look him only as a classic rider, you need to know that he won the biggest classic of them all three times in a row, plus two monuments (so 5 big ones, not "only two monuments"!), plus 3 GW, E3, Montreal, 2x Quebec, European Championship...
And third, Sagan was a top rider of the peloton, or among few of them for at least 5-6 years. He was a reference point in the races he entered, a patron of the peloton.