While understanding you can't go back and change things that have happened, acceptance that umpire error is just part of the game is a pathetic. It is a completely fixable and completely unneccessary part of the game.
Some of the arguments against are pretty lame as well, such as length of game. Expanding replays to a limited amount of on-field calls wouldn't add more than ten minutes to a game. These are the simplest kind of calls in all sports. Either the ball got there first or the runner did. Somewhere along the line 2 hour games have turned into 4 hour games but MLB has done next to nothing to change it yet wants to use length of game as an excuse to not drag itself into the 21st century.
Personally, I'd go further and take balls & strikes away from the umps. Some of whom are professionals but just as many are more than willing to change the zone per their whim, tightening up during certain times of the game, for certain pitchers, because they feel the game should be coming to a conclusion, because they are tired or hung over. A lot of these guys have egos that end up swaying the outcome of the games in subtle but decisive ways. If thats part of the game, its the very worst part.