Anyone saying he has more value in a trade is silly. "Need him resigned' is the only option. If he doesn't want to, then the Rangers are forced to trade. Otherwise you sign him and keep control of the asset.
Look at SJS or TML, they've both chosen to extend players, then it was rumored they were available for trade. Maybe it doesn't happen year one, but Staal signed long term is more valuable via trade then as a rental. Further, even with as much of a stud that I believe Brady Skjei will be in the NHL, he's not ready to take Marc Staal's spot, yet.
A few years back the Rangers were packed with defensive youth on the blueline. I advocated trading Girardi and selling high on him because the Rangers had such a dirth of young talent on the blueline. Just a season or two later, Sauer was gone due to injury, Staal was out long term with injury, MDZ was struggling mightily, and McIlrath was slowed by injuries. If the Rangers dealt Girardi then, they'd be buried. It'd be silly to trade Staal now before Skjei has a full season under his belt.
Ideal situation is to extend Staal, give Skjei a half season, minimum in the AHL to adjust to the pro game. Then call him up to fill injuries. If he advances on a McDonagh like schedule, which I believe he can defensively. Then deadline in Staal's extended year or the following offseason, trade him for max return.