I agree with you that the longer one waits, the harder it becomes. But that absolutely does not mean that players should stay in school if they get a favorable evaluation. Getting to the league a year early allows for not only a year's pay that most likely would not be made up by staying in school, but one also has access to, in most cases, better coaching, better trainers, and more time to hone their craft. It also gets one to that second contract earlier, which for the successful player is much more lucrative.
There are situations that dictate returning to school for another year, but getting one's degree is not a huge factor for most. The adage that you can always get your degree later, still applies.
You make a good point about the likely hood of going back to school once a player leaves. My stance has always been that the NFL should do more to encourage players to go back and finish. Having a plan in place to complete one's degree should be emphasized by the both NCAA and the NFL.
Btw, having a college degree doesn't assure an NFL player won't be one of those bankrupt statistics.