Hornqvist and Kunitz really aren't comparable, Kunitz fell off because his skating and hands completely disappeared. Hornqvist was never a good skater and his hands have always been mediocre. Hornqvist probably wouldn't fall off in 5 years, because he doesn't play a style of game that should deteriorate with age. He's not flying around, hitting people and taking 1-timers from Crosby on the rush, he's getting dirty in front of the net and getting those garbage goals.
With that being said, they shouldn't give him a 5 year deal, though. It's just an unnecessary commitment to make to him. I'd give him $5 million at his choice of 3 or 4 years. If he wants longer, I'd probably walk away. I do not view Hornqvist to be important enough to potentially impact re-signing guys like Guentzel, Murray and Sprong down the line, nor do I think he'll be needed in the last 2 or 3 years if he'd get a 5 year deal. At that point, the Penguins should have a replacement for him, whether it be someone like ZAR, Bjorkqvist or someone from outside of the organization.