The Pens are going to have tough decisions to make after next season. I have a hard time believing that both Malkin and Letang would be brought back. Maybe one or the other, but unless they both take VERY cheap deals, I don't see it being viable. I think this year and next are the last hurrahs for this core before the inevitable rebuild begins, or at least a serious re-tooling that involves one or more core players leaving.
That being said, I think Malkin is likely to have a shorter shelf life than Letang. Everyone always says "when Letang's body gives out, it's going to be ugly," but the fact of the matter is that's already happening to Malkin. I would be shocked if he ever plays a 70+ game season again. He has clearly lost more than a step. Even after he "got it together" earlier in the season, he wasn't even really close to a point per game. He's well into the back nine of his career and I don't know if it would be wise for the Penguins to keep him around for another contract unless he's willing to sign for well under market value and play a diminished role.