Where ya been the past year, Chico?
MLB already adopted instant replay for just those kinds of outfield boundry calls- last year!
The current replay rules allow umpires to review potential home run balls- like balls hitting near the top of the fence or close to some designated home run line- or the fair/foul status of balls that actually leave the playing field in flight near home run areas- like when the ball curves around the foul pole.
Before those rules were adopted- last year!- one of the big arguments was that allowing instant replay would add extra length to the games. It has not, since calls of this nature are fairly rare. They might come up maybe a couple of times a week in the hundreds of games played.
If anything, use of instant replay in these cases might actually
reduce the amount of downtime after a disputed call. Without the replay ruling, what you would typically see would be one manager calling time to get an explanation, and probably argue a bit, the umpire huddle up and discuss it and then an extended argument if the call wasn't changed. If it was changed, now the other manager is ticked off and he would come out to say his piece. Either way, the delays of the game could be lengthy.
Now, if the crew chief decides review of the play is warranted, the umpires check the replay and make their call. This totally eliminates the arguments and explanations to the coaches. Arguing is pointless, the decision is final and the game gets back to action in the shortest amount of time.
Applying instant replay to other situations creates some sticky problems in baseball. Using it for non-boundry/home run fair/foul calls or catch/no catch calls opens up a can of worms that would require rewriting whole sections of the rule book.
For instance, consider a routine fair/foul call. Suppose the umpire called the ball foul, it was reviewed and the replay showed it to be fair. What the heck would you do to place the batter and any runners? When the ball is called foul, the play is dead. Generally, the players quit playing. Allowing a foul call to be reversed could put the umpires in the position of placing multiple runners, well...somewhere. Just exactly where would be the problem.
Some of those runners might have very well been put out had the ball not been called dead! Some of those runners might have reached the next base, or beyond, if the ball had remained live. You might reverse the call, but placing the runners as to not disadvantage either team would be a nightmare.
Instant replay is common in football and they have rules to cover similar situations. Once a play is whistled dead, it is not subject to review. Football has recognized the fact that when the officials call the play dead, the players will essentially quit playing. Many of the the things that can be reviewed in football are very much like what can currently be reviewed in baseball- boundry issues (goal line, side line out-of-bounds) where the ball becomes dead or further playing action is not possible.
The same nightmare would happen in baseball if a catch/no catch call were reversed. Runners need to tag up on caught fly balls and any following play a fielder might make is wholely dependent on whether the catch was ruled valid or not. What if the umpire initially ruled "no catch" and the runners advanced. Where the heck would you put them if the catch was reversed? What if the runners stayed put because a catch was ruled? Reverse that one and you either deprive the defense of the chance to record additional outs or deprive the offense of the chance to advance the bases.
In football, the catch or no catch ruling on a pass doesn't have the same implications as in baseball. If the catch isn't made, the ball is dead. Nothing else can happen, so reversing the call doesn't disadvantage either team. Since the ball remains live in baseball, any reversal of the call could create all sorts of problems.
I can see where someday MLB might broaden the use of instant replay, but it would have to be on plays where reversal of the call would not significantly disrupt the natural flow of the playing action. Players react to some calls that, if they were reversed, would require the umpires to arbitrarily place runners or enforce extra outs.