One possible problem is if this rule is overturned then attacking teams will just kick it into the corners as close to the corner flag as possible to give the defending team a scrum only 5m away from there goal line. Obviously this is more difficult to do than kicking the ball dead but would still make the game more negative.

I do agree with Yed that they should be allowed to quick tap, this is where you would see speed demons at there best.

The problem with the "quick tap" is that most of them get called back because the refs are too slow "to get into position". Unless you improve the refs speed they will still get called back.

I agree about kicking the ball dead. I would suggest an even simpler solution - rather than on the 20, let the tap be taken from the 30. That's a pretty decent attacking platform.

Unless the attacking team is within the 20, otherwise it would eliminate a grubber on the 5th

Ball goes dead in goal when kicked within the 20 then 20m restart.
Ball goes dead in goal when kicked between the 20 and half way then scrum defence from point where ball kicked.
Ball goes dead in goal from attacking teams own half, or ball goes dead in goal on the full (from anywhere) then penalty defence from the point where ball kicked.

Having thought about this surely a kick that goes dead is punishment for inept fullback play. Saying that such a kick should be punished takes away any responsibility on the defending team to field the ball.