The Simpsons has been going for 20 years now. It began in Sept 89 and is nearly finished running it's 20th season. It's been going downhill for a very long time. The first really shitty episodes started about 7 seasons in, but most of them were still funny so it wasn't too obvious. But in the last 7 or 8 years it just stopped being funny at all.
I heard a good suggestion recently that they could revitalise the whole series if the next season started with the Simpsons jumped forward in age by 20 years. They'd be the age that they should have been if they had aged 1 year each season, so Bart would be 30, Lisa 28 and Maggie 21/22. It would be a whole new show and that way the writers could start completely afresh.