I would replace “will” with “can”. It does not always happen, and may occur in some departments but not others. There were past threads where a poster @bernie12 compared biology and chemistry courses at various universities; the content and rigor levels were not necessarily correlated to the universities’ admission selectivity. There was also this thread where a student at a highly selective university was disappointed with the math and physics courses there, compared to their brother’s “equivalent” courses at a different university: Should I transfer out of WashU?
So while it may be more accurate to say that a department’s course content and rigor will be held back if the students in the major are academically weak, it is not necessarily the case that the department’s course content and rigor will rise to the optimal level* with an academically strong cohort of students in the major.
*The optimal level would be that which maximizes learning without overwhelming the students with excessive workload or difficulty for them.