A rising number of (mainly) underclassmen enter the portal each year with a significant portion not finding a landing spot, i.e., leaving the D1 ranks prematurely (some remain in D1 as PWOs but many are just gone.) All the scholarships are used the next year, so the total percentage of freshmen must be increasing each year as the portal decreases the percentage of SO/Jr/Srs.

So the talent is being diluted from the top by NBA (and foreign leagues) and from the bottom by portal departures.