Illiquid markets, stable equilibria (that are not at global minima), and irrational decision making.

Unless your model accounts for such things, it's too simplified. Particularly when it comes to human factors. And if your model is that deficient, it's not really providing useful insights.

Modeling complex phenomena well is tough. But you need to have enough humility to understand when your toy model might be deceiving you.

If the shoe fits....