i think there are a lot more variables involved than: good enough-not good enough...
I think most people see it as simplistically as you do...i disagree. I think timing, luck, fit, etc matters...getting your foot in the door...with the right team, right scheme can make all of the difference...
imo when a player decides is important - if it wasnt they wouldnt have all of this rah-rah with the combine and declaring and pulling back out. etc...doing something big before you are ready increases chances of failure.
as JB once said - premature anything isnt good. or something like that.
but yeah - there are some players that are so talented that they are too good to fail - the lebrons and carmelos etc...and there are the players that no matter what they decide it wount matter - the spikes, etc...
BUT there are also the players in the limbo zone...where luck comes in and decision making matters. if you're on the lower end of the talent spectrum...of course you jump the second you have a chance...if youre comofrtably an NBA talent it might be different...