https://registrar.williams.edu/course-registration/first-year-students/ap-placement-guide/
Even at Williams, if you have 5s and know your subject, it adds options to skip boring basics or add a few interesting courses, which you may not take if you didn’t have space in schedule or had to pay.
If there is a fortunate offer of merit scholarship and enough AP credits at Northwestern or another T20, giving more money and time to William may not even look that appealing to all.