My thought is this. Until recently, the umpire's call stood as made, unless overuled by the head umpire. However, recently there has been a trend of several or all umpires discussing a controversial call. How much more of a stretch would it be for the Head umpire to have the replay as a tool available to settle a questionable call ? Obviously, not for balls or strikes, but for those plays where the umpire didn't have a good angle. When those umpires are discussing a call I'm sure at times they're talking about who actually saw the play and what they actually saw, and at times it's not totally clear. Unfortunately, a call right or wrong must be made. Replay would allow the umps to have another chance to correct a call with better data.
For you "purists", please don't tell me about it would ruin the game. Baseball is about tradition, statistics, evolution, and entertainment. How many rules have been changed over the years ? The argument about technology changing the game is bull. Didn't the Cubs just play day games ? Radio was the way to follow a game. Instant replay on television and at the park. Radar guns, how fast is this guy pitching, is he tiring ? Computers for saber stats ? Rules have also been changed. How about the DH, height of the pitchers mound, the baseball, etc. Marketing has been changed. Double headers have been eliminated, unless caused by rain outs. Week day games are ( except for get-aways ). Day games are now for Sat. & Sun. with the cavort of TBD. This has been done for broadcasting, not for the fan convenience. Speaking of which, most people have pay money to a service if they want to watch their team. These are just a few of the many ways baseball has evolved to what it is today. Like it or not, this trend will continue and at some point in time replay will become part of it.