I like the idea, though there'd probably have to be some tweaks. I know EA's NCAA Football had something this, where you could use recruiting resources to influence a guy on the fence to stay. Different factors could come into play when determining how likely a guy was to be persuaded to stay, like last season's stats and last season's team performance (the better year the player had, the better year the team had, the more likely he would leave early) and roster construction (as you mentioned, if there are multiple good point guards on the roster, a junior point guard would be more likely to leave).