Jean/Charlene was really the heart of the show and when she left, so did the reliability. The replacements for Charlene and Suzanne were either were either too kooky (Jan Hooks/Carlene), too bitchy (Julia Duffy/Allison), or too asthmatic (Judith Ivey/BJ). And without the balance that Charlene and Suzanne provided, the flaws in Julia and Mary Jo really became obvious. They doubled-down on how right Julia and Mary Jo always had to be. Without Charlene to calm them down and Suzanne to add the vapid humor, it really wasn't the same.

The writing for the show was starting to get bad even when Suzanne and Charlene were in it.

I read an opinion elsewhere that the writers thought they could just replace her with another naïve country girl transplanted to the city. But without Charlene’s big heart, it just became a stream of dumb hick jokes. Unlike Jan Hooks' character, Jean Smart's character was naïve but not stupid. And Charlene's endless optimism countered the cynicism of the other characters and was really missed those last two seasons.

Jean/Charlene was really the heart of the show and when she left, so did the reliability. The replacements for Charlene and Suzanne were either were either too kooky (Jan Hooks/Carlene), too bitchy (Julia Duffy/Allison), or too asthmatic (Judith Ivey/BJ). And without the balance that Charlene and Suzanne provided, the flaws in Julia and Mary Jo really became obvious. They doubled-down on how right Julia and Mary Jo always had to be. Without Charlene to calm them down and Suzanne to add the vapid humor, it really wasn't the same.

Another problem with BJ as a character you could argue, is that Julia and Mary Jo got loonier to compensate (when though, it was completely against their characters) for her character being boring. Or boring when compared to her predecessors, Allison and Suzanne, who at least, brought a sense of "bitchy conflict".

The writing for the show was starting to get bad even when Suzanne and Charlene were in it.

Agreed.

While opinions vary among fans, many argue that the loss of both actresses simultaneously created a void that the show couldn't fill. Here is how their departures impacted Designing Women:

Jean Smart's Departure (The "Heart" of the Group): Jean Smart left amicably after Season 5 to spend more time with her family. Fans often cite Charlene as the "heart" of the show, providing a necessary optimistic balance to the cynicism of Julia and Mary Jo. Her replacement, Jan Hooks (Carlene), was frequently criticized as a "kooky" or "cheap knock-off", with critics noting that the writing devolved into "dumb hick jokes" rather than Charlene's genuine naivety.

Delta Burke's Departure (The "Entertainment"): Delta Burke was fired following a highly publicized and bitter feud with the producers. Suzanne Sugarbaker was considered the show's comedic engine, and many felt the show "lost its zing" without her. Her initial replacement, Julia Duffy (Allison), was often viewed as "unlikable" and "too bitchy" by fans, failing to replicate Suzanne’s lovable vanity.

Shift in Character Dynamics: Without Charlene and Suzanne to ground or distract them, the remaining original characters, Julia and Mary Jo, were often perceived as becoming more judgmental and "loony" to compensate for the less dynamic new additions.

Overall Decline: Most critics and viewers agree the first five seasons were the series' peak. While the show initially remained popular in the ratings, the final two seasons were often dismissed as "laugh-challenged" with plummeting production values.

Ultimately, while Delta Burke's exit was more dramatic and publicized, Jean Smart's departure is often seen as the final blow to the show's reliability and warmth.