He fell off the cliff when he left his day job to write the comic full time. At least that is my opinion. Falling down the cliff took a while, at first he was still close enough to corporate reality to still be realistic in his exaggerations and thus funny, but the longer he was a way the less his jokes were grounded in reality and so they became not funny because they felt a little too far out.

Of course writing a comic takes a lot of time. I don't begrudge him for wanting to quit, and others have made the transition to full time humorist well - but he wasn't the first to fail to make that switch. He should have retired when he was a head....

Let the above be a warning to you. I don't know how (or if) it will apply, but think on it.