A built a framework called Lean Media for producing entertainment and informational media works (linked in my sig, for those who are interested) and did quite a bit of research on
The Simpson's early years as well as other creative groups and solo creators.
While I didn't look into these media team's later development, I think the catalysts which helped them get early success may point the way to why their later output is not so consistently impressive.
The three early catalysts are:
1. Early audience feedback loops to help guide and validate media prototypes
2. Minimal business interference
3. Really strong creative core
In the case of The Simpsons, it wasn't Matt Groening (who had previously been a cartoonist, not an animator) who single-handedly made the show. Director James L. Brooks and showrunner Sam Simon built out the team of animators and writers. They had early audience feedback from the early animated shorts that appeared in the middle of the Tracey Ullman Show from 1987-1989. These shorts were bundled into 20-minute reels that were shown to Ullman's live studio audiences while there were set changes, and they laughter and cheering helped validate the idea of doing the Simpsons as a standalone half-hour show. Importantly, once The Simpsons was launched in 1989, Brooks protected the writing crew from Fox TV executives' attempt at interference.
Similarly, for the band Led Zeppelin, the same three catalysts were in place from the start. Early creative feedback came during their first Scandinavian tour in September 1968, which directly shaped the arrangements and songs that made it onto Led Zeppelin I. They didn't even have a record company when they recorded the album with Glyn Johns that October, so the leader -- Jimmy Page -- could really follow his own production instincts.
Later on, what happened with these two teams? Certainly, they continued to make great media, but things weren't quite the same. The Simpsons team broke up -- there was a lot of tension between Simon and Groening, and Simon left after three or four seasons. Other writers went on to different shows. Fox also started to exercise more control over the production, which became more corporate.
Led Zeppelin's team remained intact, but over time productions became more bloated (LZ1 recorded in 10 days, Physical Grafitti in 18 months) and I think like many 70s hard rock groups they lost touch with their audiences. After 1977 they did not tour much, and albums like In Through The Out Door and Presence are more musically experimental and influenced by the Plant and Jones than Page and Bonham.