Bennett is 29. In his prime. He could fall off a cliff at 34. He may not. Giving him a long term deal now may have consequences later but it's a win now move.
His impact isn't just points. While I doubt Zito isn't ignoring the impact years from now, he knows the core is set to win now. Bennett even at a 6-6.5mil cap hit 5 years down the road won't hurt as much, granted the economy won't crash again and stunt the cap.
Stamkos at 34 is still a great offensively player, but his 5v5 game wasn't what it used to be, as he has had several leg injuries and has slowed down. He was still deadly on the powerplay. Giving him another mulit year deal, especially when there are signs that he may be reaching father time soon clearly spooked the Lightning to where they simply let him walk and replaced him while he was still under contract.