I don't know how I feel about instant replay but the idea that "it's been this way forever and we shouldn't mess with it" is inherently ridiculous. It's the rule (not social) equivalent of the owners in 1938 saying "Well, we've never allowed black players before, and the game hasn't imploded, so let's not mess with it." Change is inevitable, and as the game sees fit to change it should.
That said, I'm in the middle of the road on instant replay. I see the arguments on both sides (length of games too long as is vs. blown calls, especially at crucial moments) but both problems can be solved with alternate methods. Steps can be taken to shorten games, such as limiting pickoff throws, time in between innings, etc. and better, more developed training for umpires. It's hard to tell what will work out for MLB in the end but whatever they do it'll be criticized by someone. Lose-lose.