That's exactly what he does. He moves on. That's why he's inclined to put players facing questionable rehabs on IR sooner than later - he doesn't want distractions or questions or players anticipating a savior or guys assuming they're just filling in temporarily... He prepares to go to war with the troops at hand, period. He doesn't daydream about what ifs...
Brady would have been on the sideline cheerleading if Belichick wanted him to. He said no way, you'd just be a distraction and every time Cassel makes a mistake the camera will pan to you...with BB it's out of sight (and not at my disposal) out of mind.
You are mixing two points. Bill does move on but not now. Once the season starts he plays with what he has. Now he is searching every avenue to make the team better.