Electronic strike-zone calling doesn't have to threaten the labor force at all. That would be a choice.
In the post-season, MLB typically uses six umpires per game. In the regular season, they use four. They could eliminate the ball-strike judgment calling totally, put two umpires down the lines in the regular season, assign the first- and third-base umps the job of calling check swings (which they do now, on appeal), and actually ADD umpires (three on the bases, two down the lines).
All the plays at bases, balk calls, check swings, would still be handled by umps. There would have to be thought as to how tag plays at the plate (and catcher blocking) are assessed. But when you think about it, turning the "pitch in the strike zone" judgment over to a couple of lasers and a computer doesn't threaten most of what the umps do - not even all of what the home plate ump does.