I've never been a regular viewer of the Simpsons, so I have no position on this issue, but that is an odd statement. The burden of proof in this case is on those asserting that the show has declined. The default position, until presented with more information, is to have no belief.
I think that South park is a show that really improved with time, and to some degree Family Guy had a revival, but other than that, I can't really think of a show that has gone on with a high, constant level of quality.
"Some degree?" Family Guy came back after being cancelled for two years. That's pretty much the hardest degree a revival can have.
Even during the "golden years" of the simpsons the show changed, it just wasn't necessarily severe or negative.
You seem to be shoe horning some lifestyle philosophy you believe (mistrust of the "good old days") into how people receive a television series, which undeniably does change, especially one on TV as long as the simpsons.