I don't think you give it enough credit. X-Plane actually models the physical aircraft and applies aerodynamic formulas to that model. They then tweak the output for known aircraft performance. That is VERY similar to at least one level D simulator I have experience with.

Older simulators actual have very "prehistoric" code. That is why they don't actually simulate what happens when you get beyond the stick shaker. They are also worthless when they get in the flare... kind of like a consumer simulator game

What really differentiates a simulator from a game is the atmosphere. If you have a structured training program with solid instruction from a knowledgeable instructor, it really doesn't matter if one is in a certified simulator, or an FTD with X-Plane or FSX installed. If will get them 90% of the way there, but they will still need actual instruction in the plane.