Yes, the pick we received would be treated as if it was still the property of the team that dealt it. McDavid isn't eligible until 2015. The top-3 next year, at least right now, appear to be Sam Reinhardt, William Nylander, and Aaron Ekblad.
Now, all three should be very good NHLers, and on a "pure upside" perspective, all three might eventually top Stepan. However, they're not going to help the Rangers until the fall of next year at best, and for a team that's trying to win now, it's too big of a risk to take. Those players could very well end up washing out of the NHL, or, perhaps worse, Stepan could push the team he goes to into a better position and we end up losing a potential 70 point, two-way center who is captain material for the #10 and #70 picks in a giant crap shoot.
So sure, if, say, the Flames offer-sheeted Stepan and the compensation was two 1sts, a 2nd and a 3rd, and we walked away with back-to-back first overall picks (McDavid being one of them), it would be a huge win in the long run, but it hurts us in the short term and the likelihood of that happening is virtually zero.