Getting in is one thing. Staying is another. I think numbers of players in the NFL is a fair proxy for developing players. Plenty do unexpected things, and plenty wash out, but NFL teams don't have much incentive to keep players who can't play. Recent rookie contract rule changes were for this, and most money isn't guaranteed. The fact that a player is in the NFL probably means something. The fact a school gets more players there than another probably means something too. It's a good metric of program strength, recruiting, and coaching.