They'd all like to use every healthy player they have. If they have 47 healthy, they'd like to dress 47 -- as long as the other team dresses only 47, too. The rule is designed to prevent one team from having 53 healthy players, while its opponent has only 45. That's a distinct advantage for one team. Having eight inactives each game levels the field. There's not a coach who would want to change that.
Essentially, what you would want is to increase the size of teams to include 53 players, plus a short-term IR for players with minor injuries, plus players on the long-term IR. And that's a collective bargaining issue. If you add several hundred players to the league, with the same salary cap, that's more players to share the same amount of money -- not exactly what the NFLPA wants. Plus, if there were a short-term IR and no inactive list, teams would be compelled to put players on the short-term IR so the team isn't at a disadvantage in a game.