I think that's a bit simplistic. I've had coworkers that became better employees over time. The "problem" with PIPs is by the time you've screwed up long enough to be put on a PIP everyone knows there's no turning back.

For example, a friend I have that recently left Facebook knew for a good 6 months he needed to shape up. But they hadn't put him on a PIP in that time. They eventually offered him a decent severance to quit, and he took that rather than continuing to try. If he stayed, he probably would have been put on a PIP fairly shortly. It was the best thing for everyone. He wasn't all that happy there anyways.