It's boring but I think it's just a natural part of the game. Locking in a pitcher is unfair if the opposing manager can still pinch hit. Of course they could outlaw pinch hitting too, but then I'd ask how many rule changes do we need to go through for the sake of cutting off a few minutes of down time?
There's changing rules to make the game better (like replay) and changing rules to make the game faster. If we really want to make the game faster without tampering with commercial breaks or actually enforcing the pitch time and batter's box rules, then we have to be prepared to degrade the actual strategy and intrigue of the game.