Honestly, I think the majority of the umpires do a rather solid job calling their games. And I think we all know which ones are not in the majority, but even those we umpires might think are subpar, are probably still getting the vast majority of their calls right.
I think there are two main problems. One is that there are a LOT of egos out there wearing blue. I recognize that it takes a crapload of hard work to get to where they are. But a lot of them seem to have a chip on their shoulder. Not the majority, but enough to have created a
PR problem. The other is that TV, with HD and 24-32 cameras in a stadium, the errors that ARE made are more public, more glaring. I would be you that umpires are not any worse than they've been over time (and quite possibly better). But now that people upset about a call can see it in ultra slomo to "prove" they are right, their complaints are more vocal.
My worry is that the perception (true or not) that the umpires are worse than they used to be, coupled with the arrogance and "above it all" attitude of many of the umpires, and even of the procedures of the group of umpires, is going to lead us to a significant increase in replay usage.