I think that is incredibly deceiving though.
Look at last year's draft. There were 254 overall draft picks, of which 184 came from the "automatic" conferences (roughly 72%).
In those six conferences, there were 68 teams (2012 since this was 2013 draft). If we assume that each team was at the full 85 limit and that 25% of their players were seniors, that means there were roughly 1,445 players exhausting their eligibility in those conferences. That translates into roughly 13% of the players making the league. I know this is a crude way of doing it, but I think it is directionally correct. Obviously, when you start taking away the schools that are in these conferences but are not traditional powers, that % increases even more (TTU, Kansas, Kentucky, Wake, Duke, etc). Point being, that I think it is safe to say that 15-20% of the prospects brought into schools like ND, Bama, Florida, OSU, MSU, UCLA, USC and the like end up being drafted. Losing those players from those schools will make the product on the field noticeably different IMO. Not saying it would or not would be watched as much, just watered down a bit.
2013 NFL Draft by conference: SEC doubles the competition - CBSSports.com