Why do all TV shows eventually suck?

Problem: Why is it that so many television shows start off well and then begin, slowly yet inexorably, to suck as the years pass?
Answer: Good television never happens on purpose. It is always an accident, one which market forces will sooner or later correct.

This is how The Plaid Adder opens his theorizing on The Accidental Theory of Television. I don't agree with it as much as his other "crackpot theories" on the same page...but it made some sense to me.

Please read the whole thing and see what you think.