When did it lose the spark - (Preview "The Simpsons")

It has been a long while since the Simpsons debuted on Fox (Ya I know really it was the Tracy Ulman show but fuck it). Way too many Seasons later and it's still going on. Just how long is too long to keep up good writing. How long can you stay relevant before the viewers say "No thanks there is a talking dog and a baby on later". How many products can you peddle before people say "A Simpsons shirt? They still make those?". Exactly when does what "Guest Voice" become the only reason to tune in. When did it lose that spark? Coming Soon.