The rule change is not about protecting injured players as most players in soccer don't get as many injuries as football players. The fouls are for playing a player, (trying to kick/push the player vs. trying to go after the ball), instead of playing the ball, 2 yellow cards kick a player out of the game so 5 subs vs. 3 subs does not stop injuries.
What this rule change does is it changes the strategy of changing the make-up of type of players on the field. (think offense or defense)
In the past, traditionally, if a team has a lead and it gets to the 3/4 ish point of the match, that team may take out a forward, an offensive player and replace them with a defender, to help stop the other team motivated to tie the game up as the time is running out.
And, there is also the strategy of at what point in the timeframe of the game do you insert your new players, because these 3 players coming off the bench, if used, often/always have fresh legs, meaning they haven't been running back and forth for the previous part of the match and therefore move around faster than the tired players, and impact the game significantly, usually.
Therefore, changing to 5 substituions from 3, for the game total, is going to redefine what the game of soccer becomes, (It cheapens things by giving less weight to the decision to make a substitution)
at the expense of what it has always been, and it won't be the same game.
This is kind of like adding a designated hitter to the national league in baseball. It will tear at the fabric of what soccer is and how it is played as a game, and having no standards against that means, what do you have?
Bowling with I3 pins? What do you have? It is not soccer. It is not soccer as it should be. It is something else, it will become something else.
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It would be like adding a 5th down in football, or,
or, It would be like allowing pass interference penalties to go to the I yard line and be 50 yard penalties instead of I5 yards.
But moreso:
It's like adding a 5th down in football, an extra chess move, more pieces to move backwards in checkers, bicycle races with motors attached, that sort of thing.