The issue is that I have seen players flagged for UNR for tackling the holder. The problem is there really no way to tackle a guy on his knees without it looking bad. If there is going to be an exception to the normal rules of when a player is down, the rule should explain how to end the play during the exception period. Tapping the guy on the shoulder is as good a way to guess what makes the player down as tackling a downed player. If you are going to have an exception, you have to explain what makes the excepted player down. Otherwise, we are all guessing.
The exception allows the ball to remain alive while he is holding/controlling the ball for a potential kick. As long as he is still holding/controlling the ball, and a potential kicker is in position to make a kick, the ball remains alive, even if an opponent manages to get to him in time to contact him before the ball is kicked.
Let's assume an opponent gets to him in time to contact him before the kick. What is likely to happen?
a) He'll maintain his hold/control of the ball and the kicker kicks it. This is just football. Play on.
b) He'll maintain his hold on the ball, but move it from its position on the ground, meaning he is no longer holding/controlling the ball for a potential kick, and he is now a ball carrier. As long as he WAS legally holding/controlling the ball for a potential kick, he may rise and attempt to advance the ball, or throw it. The ball is still alive. If an opponent tackles him such that his forward progress is stopped, then the ball is dead. As Bossman said, most likely some other body part will touch the ground, and that would qualify to make the ball dead. Or, he'll rise from the ground (ball still alive), then get blocked back to the ground - ball dead.
c) He'll lose his hold/control of the ball and the ball will be loose from a fumble. Still alive, but, loose.
This isn't touch football. Just touching a legal potential holder won't cause the ball to become dead. Yes, the potential holder is at risk of getting a vicious hit. Targeting and personal foul rules still apply. But a good, hard tackle/block to the body with an opponent's shoulder/body is just football. If that happens after the ball is legally kicked, then it becomes running into/roughing.
The rule is good as it is.
Robert