perico is spot on, and again, having followed the sport since the mid-70s and big time since 1978, I can add to that post. I liked Sean Kelly. After him, only one rider had a somewhat similar set of skills: Jalabert. Both excellent sprinters but not top-3 in a GT field, multiple green jerseys, only a hand-full of stage wins. But always scoring points, and scoring on medium mountain stages when the pure sprinters survive in the gruppetto.
Kelly was a bigger guy than Jalabert. That's the difference, one climbed a little better, the other performed better in tough-guy monuments like PR or RVV.
Sagan is not good enough of a climber to be compared to Kelly, Valverde is not nearly as good of a sprinter, riding styles are different, and as perico mentioned, designs and tactics were different.
And so were salaries: you couldn't just race in July in Kelly's time in particular. Money and specialization pretty much came with Tapie La Vie Claire and LeMond's contract. So in addition to the likes of Moser, he would face a hungry and angry badger in the monuments. Jalabert still faced big-time foes, but the field in the one-day races was thinner in my opinion.
Both great, edge to Kelly.