The problem with your line of thinking is that every move in baseball is affected by a counter move. You can't look at the effect on Mookie alone, but rather the effect that moving Mookie has on the entire lineup. Sure, if Mookie were batting 3rd, he'd likely have more RBIs. OTOH, he would definitely get less at bats, the lead off spot, which comes up most often, would be weakened (which is no small thing), and Mookie would likely score less runs. The net effect would be a negative one, if anything.