Really? 12 years is smarter than 7 years? Pronger has proven he can play at a pretty high level at 35 years old, when we have no idea how good Zetterberg will still be 35, when they are only halfway through that contract. A lot can happen in that time, you are correct.
As wr50l pointed out, the Pronger deal gives the Flyers a great deal of flexibility. They aren't hooking themselves up for the whole time with no way out. This deal likely gives them more flexibility than a 3 year deal for more per season would.