For most colleges the number of available slots is not constrained by some absolute limit but is instead constrained by their income available to pay for all the people and resources needed to educate these students.

Further, the most competitive universities artificially constrain acceptance rates because low acceptance rates make them more desirable.

Imagine we passed a federal law banning the children of parents who make more than $150,000 annually from attending college. Would this just mean that colleges take their same planned slots and give them to lower earning students? No. It'd be massively disruptive and change the available slots.