The primary reasons for the extra subs was the level of injuries. With the modern game being played with its current levels of pace and physical impact,
there are many positions which really cannot play the whole 80 minutes (scrum half, hooker and openside flanker being three of them). No matter how fit the players are, there are limits to human endurance and fitness; players will tire toward the end... that might good for the fans but it is bad for the player... tired players are far more susceptible to injury.
If they go ahead with reducing the bench to only five, with three having to be props (that is compulsory) then that really leaves two to cover 13 positions. One will have to be a hooker, so that is one player to cover all the back positions, the loose forwards and the locks. You can be 100% certain that player injury rates will skyrocket, especially in the grass roots game.
What I would rather at the top levels is the following
1. a bench of 10 players, with only five tactical substitutions allowed (plus two substitutions reserved for props only)
2. unlimited replacement of genuinely injured players (within those 10 named players on the bench) but the injury must be certified by the match doctor (not the team doctor)
3. a player failing an HIA can always be replaced, but the replaced player is stood down for four weeks (stops teams gaming the system)
4. Blood replacement rules remain as they are
As for the 50/22 law they proposed, well that comes straight, from RLs 40/20. I proposed this idea about 9 years ago, but as a 10/22 (own 10m to opponents 22) and was scoffed at.
https://www.therugbyforum.com/threads/the-kicking-problem.15821/