While this is true, if players weren't benefiting from college, more would declare early for the draft, after 3 college seasons. I don't know what the percentages are, but there are more players getting drafted after 4 years of college than those drafted after 3.

Which implies that players not only seem to value the training and experience of college athletics, but that they value all 4 years of it. Arguably red-shirting extends this to 5 years.