Does baseball really have an audience problem? I hear that a lot, but the truth is, they reap more and more money every year, and the growth is huge, not slow. (Which is why player salaries are heading to the moon.) Commentators seem to think the games are too slow and take too long, but I don't hear anyone suggesting they cut the length of commercial breaks. Those are what's driving the time issues, but of course, they also drive the $$$. In any event, I don't hear anyone who paid hundreds of dollars to take their family to a game saying, "You know, I just wish I'd only gotten 2 hours of entertainment for my money."
That being said, I long ago got my fill of umpires who like to talk about "my zone." It's not your zone, buddy. There's a rule book and you're being paid to apply it. No one asked you to put your own personal spin on it.
So, I'm all for the machines. A simple ding sound would signify a strike and the call could be quickly posted to the scoreboard. Something similar for balls.
You'd still need an HPU because someone has to call plays at the plate, fair/foul, and many foul tips, HBP's, catcher interferences, etc. Plus someone has to step in if the machine goes down.
My only caveat would be that we need to get straight how the machine sets up its box. The sides of the plate are stationary, but the upper and lower limits have to be reset with every new batter. If you read the rule book and then look at K Zone on TV, the K Zone box cuts off a fair amount of the upper strike zone. I don't know why this is or who's programming that thing, but they should be fired. Right now, the upper line tends to get set at the belt buckle, and that's not what the rule book says. I don't remember anyone telling the K Zone people, "Ignore the rule book and keep doing what the umpires have been doing." I thought the whole point of K Zone was to push the umps to get back to the rules as published, but at the upper limit, it's not happening. Not yet, anyway.
I could imagine each jersey having a chip in it that communicates with the system as to where the upper limit is. Maybe another chip in the pants for the lower limit, too. Monkeying with your chips would be like using a corked bat in terms of ejections/suspensions.
BTW, if you think MLB is turning into home run derby and not enough of the scrappy play that I personally enjoy, calling the high strike is the ticket. A batter who has to protect against the full height of the rule book strike zone will have a lot of difficulty using today's typical MLB "launch angle" swing as his standard approach. That uppercut path struggles with higher pitches, which is why batters carp every time an ump calls anything above the belt -- they can't hit it, so they act like it just shouldn't be called.