The difference with RL is that they can afford this because the defending backline is 10 metres away, and the defending markers (the two opponents marking the player playing the ball and the dummy half) must be "square" that is, lined up directly ahead of the play-the-ball and behind one another parallel to the touchline ("markers not square" is a PK in RL)
In RU, the defending backline is only one or two metres away, depending on how "deep" the ruck is, and in the modern game, rucks are very shallow because teams do not commit more than two or three (often less) defenders to the ruck. Consequently, the defending line is very close to the opponent's HMF.
Were we to apply your suggestion, the acting SH would probably be scragged more often that not when he tried to clear the ball, then what sort of game would we end up with? Well, I guess my suggestion was poor. The ball would have to end up out, but not necessaryily far away akin to RL.
What you suggest worked in the past for two reasons
1. Rucks were much, much deeper, with more players committed by both sides, so the defending line was further away, and You mean a smothering defense. This should only change attacking strategy i.e. more kicking not law.
2. Players were less fit and considerably slower, so the acting SH had more time to clear. Shouldn't the SH also become quicker and adapt to the game instead of the laws/clarification adapting to them. Winning ruck teams could also invent alternate methods for the SH to get the ball. I fear we (refs) are adapting law to fit players or the perceived spectators joy and their money.
I mentioned that RL has the defending line 10m back. It used to be only 5m until about 20 years ago. The reason that it was changed is was precisely because they recognised that players were becoming faster, fitter and stronger, and that was resulting in it becoming too easy for defending teams to stifle the attack. Oddly I was stationed on a peacekeeping force about 20 yrs ago and the Kiwis were always undecided whether to use 5 or 10 m on Sundays. Saturday was Union day.