I would agree that the top 150 players are better than the next 150 players but that is likely because a minority of those top 150 players are much better than the rest. I don't know that I can agree with the "overwhelming majority" part without some clear empirical demonstration of that assertion which I'm pretty confident that neither you nor anyone else has. If one cares to take the time, it shouldn't be too hard to find quite a few players who were not in that top 150 who had better college careers than quite a few who were ranked somewhere in the lower third (or even lower half) of the top 150.
Rating recruits isn't a science - it's an art. There are no qualifying board exams, no controlled experiments, no authoritative literature, no set group of researchers always doing the scouting and rating, and what's most important - no legal liability for professional failures. Programs like Kentucky and Kansas that get the pick of the top 50 can pretty much ignore unranked players and feel pretty secure about it, but programs below the most elite tier probably shouldn't.
