Of course laws can overturned by Congress, or just not signed by the President in the first place. But those not the only check on the enforcement of unconstitutional laws. That's where the courts come in. They were not lofted above the Constitution, but rather are supposed to enforce it. Even if the people support such a law, it is not allowed, but in such an instance, the Congress and President, rather than having an incentive not to pass it, have every incentive do so.
We are are a Republic, with a written Constitution. Depending soley on what are in essence "democratic" forces to enforce the Constitution is a losing proposition.
Agreed. Which is why the Constitution may turn out to be a suicide pact after all.