Doesn't the process of a throwing motion take only a second? Less so really - yet there appears to be no trouble with that. If you allow the refs to determine when a QB's arm is going forward, what is wrong with letting them determine when that throwing motion is complete or stopped? Granted, there will be close plays - like there are when a QB raises the ball and gets hit and it isn't clear if he was going forward or not. But that is preferable to a rule that says the ball has to be extended at half an arm's length or something like that to be considered a pass - which is equivalent to setting some arbitrary standard that the ball has to be tucked next to the body in order to complete the throwing motion.
The cases where the tuck rule has been applied that I have seen all are pretty clear that the QB is not throwing the ball - he is bringing the ball towards his body, not going forward. If you allow refs to make the judgment that arm is going forward on the one hand, why disallow them to make the jdgment that the arm has quit going forward, which is what the tuck rule does.