It's weird to say, but The Simpsons is just as old and iconic as Dragon Ball Z. Both aired their first episodes in 1989, 30 years ago (though, obviously, Dragon Ball is older since the manga started in 1984 and the Dragon Ball anime in 1986). The two are undoubtedly among the most influential pieces of media ever created.
However, for better or worse, over time the two series have reversed. The Simpsons was, at one point, critically lauded and praised. Dragon Ball Z, on the other hand, was a popular but often mocked series, with many dismissing it as just explosions and beam fights. But over time, The Simpsons has become a joke, while Dragon Ball Z has been elevated to modern mythology.
Why Were They Special?
To understand the series' legacies, it is vital to understand why they stood out in the first place. No, The Simpsons was not the first cartoon for adults (Fritz the Cat and the rest of Ralph Bakshi's filmography existed long before The Simpsons). No, Dragon Ball didn't invent the shonen genre (Astro Boy arguably started that). Nor, honestly, are either the best in their respective genres. Later series like Futurama, South Park, Rick and Morty, One Piece, Rurouni Kenshin and My Hero Academia are arguably improvements of the formula.
What they both did do, however, is create the tropes that everyone who followed copied. The Simpsons took biting social commentary and mixed it with the already cliched sitcom structure, using animation as an opportunity to explore that genre's humor in a more meaningful way. Dragon Ball Z, on the other hand, took the action anime genre already established in prior manga (and in Dragon Ball itself) and just amped it up to 11.
They weren't the first in the genre. They were just the first to find that perfect balance.
So, What Changed?
Both series arguably lost steam as time went on. Most people will argue that Dragon Ball went downhill after the Cell Saga. The Buu Saga is the weakest of the three core eras of Dragon Ball Z, with Dragon Ball GT being a general disappointment and Dragon Ball Super, while not bad, being a step down from Dragon Ball Z.
The Simpsons, on the other hand, peaked around Season 8 or 9, but, when most of the writers started to either move onto other projects or just leave, the show's jokes grew stale and repetitive. The characters lost their appeal as they were forced into convenient tropes that remained overly consistent.
Additionally, those influenced by these two shows started innovating on the formula.
Both shows declined, but one declined by recycling the original, while the other continued building inferior but admittedly entertaining content.
The Simpsons is Stagnant
The problem is The Simpsons never changed. It remained the same thing, time and time again, to appeal to the same demographic every year. Its humor never evolved for the times, beyond the increased quantity of celebrity cameos.
However, Homer and Marge's relationship has not changed in 30 years. The children aren't getting older. New characters are rarely introduced, while older characters are "Flanderized," a term that can be traced back to The Simpsons, thanks to how the character Ned Flanders, once well-rounded, became defined by his most popular gags.
This stagnation resulted in an overall boring product. While episodes can, on an individual basis, be entertaining, there is no sense of progression watching the series. There is no reward for continuing to watch the series. Long-running shows can only last so long with the same plots, same jokes, same characters and same scenarios before people start losing interest, especially when each new joke is less funny than one from four seasons prior. About the only thing The Simpsons has to offer is the inevitable Treehouse of Horror episode.
Dragon Ball is About Progression
Ironically, at its core, The Simpsons could never have lasted as long as it did. The sitcom style it parodies always had an expiration date. On the other hand, Dragon Ball Z is built on a different foundation: progression. By its very nature, each episode will push the narrative, characters and story forward.
One of the key reasons Dragon Ball GT failed to satisfy its audience is because it tried too hard to return to the era of Dragon Ball. However, by turning Goku into a child and focusing on colorful misadventures, the series countered the fundamental quality that kept the series exciting: progression.
Goku starts the series as a kid, but he soon grows up. He gains abilities, meets new friends who grow up with him and clobbers increasingly powerful adversaries. One reason the show has remained eternally fresh is because it continually offers fresh ideas. There's a reason why Beerus and the new version of Broly have revitalized the show. They offer something new to push the series onward.
The Simpsons needs to remain static in order to fit in its box. Dragon Ball Z doesn't.
It's also for this reason that video games based on Dragon Ball Z remain popular over a decade after release, while most Simpsons games feel like duplicates of whatever was popular in that era. The Simpsons is dead because it outlived its lifespan. Dragon Ball Z reinvents itself every few years.