I fear that if the rule is changed upwards (like some people want), you would see more players enter the D-League straight out of high school or play abroad in lieu of the NCAA. While this might be a result the NBA would prefer (Adam Silver has mentioned that he wants to bolster the D-League anyway), I think it would make the college game worse.
I suppose the NBA could also change its eligibility rules for athletes coming from the D-League and abroad (to match an equivalent increase in eligibility for those coming from college) to try to avoid such an outcome. I suspect however the lure of being paid (in the D-league or abroad) would be too much for many *top* players to reject. I think until schools provide *top* athletes with something more valuable for their services, it will be difficult to prop up the college game up any more than it already is without violating laws against collusion (between the NBA and the NCAA).
Although I don't support doing so, I think increasing the threshold to two years would be less catastrophic to the college game than increasing the threshold to three or four years.