I don't see why we should care.
Kessel is going to be scary, but he isn't the whole team. Ryan Potulny looked scary for the first half of the season. Second half, not so much.
Teams adapt to learn other teams' strengths. Kessel will be scary for the first half of the season. He'll still be scary the second, but not as much. We'll learn how to counter him.
The same thing was said of Earl. No one could stop him early in the season.
Vanek was a good player offensively. I want to see how much of a two way player Kessel really is. If he can play defensive forward as well as his offensive skills seem to be, then I'll be worried.
Remember, one player doesn't make the team.