All human umpires tend to have consistent blind spots / biases among them. (You can read the study link for more details.) The rule book used to say the top of the zone was the letters, but every player knew that pitches at the letters weren't called strikes very often -- in part because it was an ump blind spot (it's hard to judge varying tops of the zone, with different batter heights/stances), but also because batters didn't like those pitches and, over time, had taken enough of them to communicate that. Eventually the rule book zone was amended to reflect this. Player training and decision making is based on this kind of reality, rather than strictly the rule book. If it wasn't, I agree it would be a serious problem -- there would likely be chaos in the game! But there never really has been, and to the extent that there ever was, the Eric Gregg game was 22 years ago. Tom Glavine retired 10 years ago. I rarely see any ejections over balls and strikes. Most excesses of the umpire-player relationship have been curbed. They don't always agree, of course, and players wouldn't always agree with Statcast/robo-umps either, but the game is stable. I think it helps that players are dealing with the same kind of human limitations as the umpires. Joey Votto has a great eye, but even he probably couldn't call a zone with an error rate significantly less than 9% or whatever, as compared to Statcast. Fortunately, he doesn't have to, to be successful. His eye can provide the 9% zone, his brain can quickly judge the risks/rewards based on factors like the count, and his reflexes and muscle memory regularly allow him to hit the ball well too. I guess he could follow the same basic formula under robo-umps, but at the same time, I don't get the sense that any of that formula is really being compromised under human umps.