The Horrible Aftermath:
First of all, we're explicitly told that the energy produced by laughter is "10 times more powerful than screams." That's all well and good, but the monster world doesn't exactly appear to be in the grips of an energy crisis. A sudden 1,000 percent increase in energy efficiency means that you can lay off 80 percent of your workforce and still double your output, so the world of Monsters, Inc. is about to have an unemployment rate higher than any Third World country.
Pictured: An unsustainable growth curve.
Secondly, it takes absolutely no effort to make children laugh (for example, Cars is far and away the most successful Pixar movie, and Cars is about as funny as a dead clown). At the end of Monsters, Inc., Mike gets his entire daily quota of laughter from a single child just by delivering a mighty belch. That is the very definition of unskilled labor. Anyone who graduated from Monsters University now has a degree in scaring that isn't worth the paper it's printed on, since all it takes to throw kids into fits of hysterical laughter is a long, whistling fart.
Or a fart that just got entirely too real.
And all of this is in a world where scream production was the center of their economy -- imagine the Middle East if overnight somebody invented a technology that rendered oil worthless. Sure, energy is suddenly cheap, and that'll surely make for some lower air conditioning bills, but that's not much of a silver lining in a devastating economic depression. A crisis that, by the way, will soon flood the streets with thousands of unemployable monsters, each harboring a bitter grudge against human children.
"... and the human-hating racists had access to the children's bedrooms forever after. The end."