Was "Marge Be Not Proud" the beginning of the end for The Simpsons? Everybody knows The Simpsons is not what it was. It hasn't been for almost two decades; we're much further from The Simpsons Movie than The Simpsons Movie was from good Simpsons. This can be easily seen by firing up any classic, seasons 2-8 episode and comparing it to what's produced now; season 30's currently airing and, while it may not be quite as nonsensical as the late teens/early twenties, the bite and verve is so removed they barely qualify as the same show.
If you want to learn more about how this happened, you can't find a much better account than Charlie Sweatpants' "zombie simpsons: how the best show ever became the broadcasting undead" on Dead Homer Society. It's a 22,000-word mini-book that charts the show from industry-altering rogue to unwitting status quo to shambling husk of its former self, with anything post-season 10 humorously dubbed as an entirely new show, "Zombie Simpsons". The essay covers everything: the innate satire in The Simpsons family concept, the writer turnover between seasons 5 and 8, and, most in-depth, the season 7-10 episodes where the cracks begin to show. And while typical punching bags - "The Principal and the Pauper" chief among them - get their fair shake, that's not the first sign of rot highlighted.
That would "Marge Be Not Proud". The Simpsons season 7, episode 11 first aired on December 17, 1995 (exactly six years after the series premiere, "Simpsons Roasting On An Open Fire") and explores a mother-son rift: Bart wants new videogame Bonestorm and, when Marge refuses, he tries to shoplift it but is caught; when the truth comes out (and the family's Christmas photo is ruined), Marge becomes emotionally cold to Bart, leaving him adrift until he learns his lesson and presents her with a paid in full portrait (she gets him the videogame "every boy wants", which is the dry Lee Carvallo's Putting Challenge).
By most accounts, it's a good episode of The Simpsons that explores a lesser-trodden relationship among the core five and is full of classic moments: Thrillhou, capice/catfeesh, "I don't remember saying that" and Lee Carvallo's Putting Challenge are regular rotation Simpsons memes. It's a favorite of BoJack Horseman creator Raphael Bob-Waksberg, who told Rolling Stone the emotional balance influenced his Netflix series.
But Charlie is not a fan. Citing the family sitcom trope of a "Very Special Episode" (where drama takes a backseat to exploring a socially-relevant issue), he argues that this saw The Simpsons step away from satirizing television norms to following them. He's not overly wrong either; whereas previous brushes with issues saw The Simpsons take down the institutions advocating good as much as the problem this one is more binary and sees Bart actually learn a resolute lesson that almost goes against his character. For all the video game jabs, the primary narrative lacks the subversion element that made the show work: stealing is wrong, period. There's no gag like there was in "$pringfield" or "Bart Gets Famous" with unbiased takes on legalized gambling and showbusiness respectively, nothing to distinguish the plot from the more standard comedies of the time.
However, while all that may be true, it doesn't make "Marge Be Not Proud" a bad episode. Just as the push by writers in season 4 to make Homer stupider (a trend so established it was lampshaded in "The 138th Episode Spectacular") created some of the best episodes before eventually birthing an unlikable buffoon unaffectionately called "Jerkass Homer", so too is it less the episode itself as much as what it represented. The Simpsons would soon begin having to target itself - see season 8's "Itchy & Scratchy & Poochie" and "Homer's Enemy", and eventually, season 12's "Behind The Laughter" - but before it was the status quo to lampoon, it had begun to morph into what it never wanted to be.
So, "Marge Be Not Proud" exists as both a solid season 7 episode entry but also an unwitting sign of things to come. The Simpsons lasted too long, plain and simple. What the Dead Homer Society's essay does is highlight how the things that would eventually undo the show were there from a lot earlier than we may realize.