I think the big board formula is very different from how the draft works in real life. They REALLY undervalue freshmen in the high end of the draft relative to how real life works, which probably makes the draft work better.
If we are looking at upperclassmen drafted in the top 10 of the past actual NBA drafts, we get:
2016: 5 freshmen, 3 upperclassmen, 2 international
2015: 5 freshmen, 2 upperclassmen, 3 international
2014: 6 freshmen, 3 upperclassmen, 1 international
2013: 3 freshmen, 7 upperclassmen, 0 international
2012: 5 freshmen, 5 upperclassmen, 0 international
This years draft big board obviously varies by site, but id bet that most of them have a very similar top 9 in Fultz, Ball, Jackson, Tatum, Isaac, Monk, Smith, Fox, Ntilikina which are 8 freshmen and 1 international
vs WIS where the highest freshman in my worlds are:
29 in tarkanian
there are none in the top 100 in Rupp
44 in Crum
38 in Allen
With the EE *ahem* issues, i can see why the value of freshmen is so low on the draft board just for making the game work... but conceptually its an interesting thought to picture how the game would change if they made it actually like real life where the top 10 prospects are all EE's