The Walking Dead's decision to kill off Carl (Chandler Riggs) allowed for Negan's redemptive arc in seasons 9 and 10, but this wasn't a good enough reason to kill off a character who had been on the show since the beginning of season 1.
One of the most shocking moments in the history of the series came in season 8 when Carl lifted his shirt to a distraught Rick (Andrew Lincoln) and revealed that he had been bitten by a zombie. Carl's fate in season 8 became the show's most significant death, and it still has yet to be surpassed. Carl's death had a tremendous impact on several of the characters, particularly Rick, who spent several episodes reeling from what happened. Carl, who spent much of the season talking about how he wanted people to live in peace and find a way to move forward, had a posthumous effect on Rick, his outlook on life, and his leadership style.
It was confirmed in interviews that Carl's death served a very specific purpose. It didn't happen because Riggs wanted to leave; it happened because of the emotional impact it had on Rick. It's been explained that Carl's death needed to happen to get Rick in the mindset of his comic book counterpart. In the comics, Rick chose to spare Negan's life at the end of the All-Out War, but in the show Rick was headed down a different path. With everything that Rick was doing, it was hard to imagine Rick not killing Negan. Based on his behavior throughout the season, such a sudden shift would have felt out of character. But Carl's death changed that; inspired by Carl's ideals, Rick allowed Negan to live.
If Carl hadn't died, Rick would have undoubtedly killed Negan without hesitation, and most of the characters on the show would have accepted that decision. However, this would have cost The Walking Dead several key storylines from the comics. After the All-Out War, Negan went down a path of redemption and continued to be a popular character.
Essentially, The Walking Dead showed that it was willing to kill off Carl to keep Negan, but was the trade-off really worth it? It's true that Negan is well-liked by fans of the comics and the show, but as everyone associated with The Walking Dead continues to reaffirm, the show doesn't have to follow in the footsteps of the comics. It would have better to let the series follow its natural course. If Rick was poised to kill Negan, The Walking Dead didn't need to kill off one fan-favorite character to prevent another from meeting an early demise.