Ordinarily, showrunners get to determine the trajectory of their TV shows from the pilot to the finale. On rare occasions, some are forced to leave midway, either due to personal reasons or creative differences. While showrunner changes don't affect some shows that much, other shows suffer a decline in quality.
Examples of shows that got better include Star Trek: The Next Generation, which was salvaged by Michael Piller as well as American Dad, which stopped being too similar to Family Guy when Mike Barker and Matt Weitzman too over, Sadly, for a couple of shows, change was a terrible thing.
Mayans MC (2018 - Present)
After the success of Sons Of Anarchy, Kutt Sutter was tasked with transferring the same magic to the spinoff Mayans MC. Unfortunately, Sutter was fired after Season 2 after accusations that he was mean to cast members. He was then replaced with Elgin James.
The fact that James is a former gang member has helped in making the FX series feel more realistic. But it appears viewers are more fond of the Sutter days since Nielsen ratings sank from an average of 1.07 in Season 2 to 0.8 in Season 3. Further confirmation of this lies in the Rotten Tomatoes audience score, which also tumbled from 82% to 53%. Even so, all is not lost. With a fourth season on the way, there is still a chance for redemption.
Gilmore Girls (2000 - 2007)
The merger of The WB and UPN to form The CW brought much hope but not to showrunner Amy Sherman-Palladino who had to leave over disagreements with the new management. She was replaced by David S. Rosenthal but standards dropped, forcing the network to cancel the sitcom.
There is clear evidence as to how Amy's departure damaged the show. Viewership figures quickly dropped from 4.5 million to 3.7 million while the Rotten Tomatoes score dropped from 100% to 67%. Luckily, the door hadn't been shut forever. After years of ruing the demise of their favorite sitcom, fans were blessed with a miniseries revival in 2016.
The West Wing (1999 - 2006)
Showrunner Aaron Sorkin had to leave the political drama after Season 4 due to differences with Warner Bros Television executives. John Wells then took over but struggled to steer the ship.
It didn't take long for Sorkin's absence to be felt with Season 5's average viewers dripping to 11.8 million from the previous season's 13.5 million. The final season had even worse viewership figures of 8.1 million. Addtiionally, Rotten Tomatoes scores dropped from 86% to 62% between Season 4 and Season 5. Consequently, the political drama only lasted 2 more seasons before coming to an end.
The Walking Dead (2010 - Present)
Scott M. Gimple oversaw the successful run of one of the best zombie shows before he was promoted to Chief Content Officer for the entire franchise. Sadly, his replacement Angela Kang couldn't replace the same magic.
Kang's two seasons in charge have had the lowest ratings, with Season 10 only getting an average of 3.04 million viewers. It was, therefore, no shocker when it was announced that the show's current 11th season will be its last. Another showrunner change would probably have salvaged things but since there are a number of spinoffs already, this seems unnecessary.
Designated Survivor (2016 - 2019)
Designated Survivor is one of few shows that were canceled and then brought back, with Netflix picking it up after ABC dropped it. The first season had Jon Harmon Feldman and Jeff Melvoin as showrunners before Keith Eisner took over. Neal Baer was then hired for the 3rd and final season.
The high turnover of showrunners saw the series morph from an espionage thriller similar to 24, to a political drama that attempted to imitate The West Wing. This didn't work as the show was canceled by Netflix too. With Season 1 getting a Rotten Tomatoes score of 87%, the second and third only managed 60% and 67% respectively.
The Simpsons (1998 - Present)
The long-running animated show has had a couple of showrunners but things really to a turn for the worst when Oakley & Weinstein handed over the steering wheel to Mike Scully. The showrunner has been criticized for being the person under whose tenure the show's golden age came to an end.
The fact that the show continued to suffer declining ratings even after Scully's departure is undoubtedly a result of audiences losing faith. Thankfully, the numbers aren't bad enough to make the network consider pulling the plug. At the moment, the staunchest of fans can be guaranteed several more adventures of Homer and his family.
Seinfield (1989 - 1998)
Before moving on to start his own show, Larry David was the brains behind one of the best sitcoms of all time. Two seasons after his departure, the show came to an end.
The fact that the show had to come shortly after David's departure says a lot. The decline in quality is evident through critical reception. Prior to David's departure, each season had managed a flawless 100% Rotten Tomatoes score but the last two seasons only managed 86% and 61%. Luckily, the plug was pulled before things got only worse.
Dexter (2006 - 2013)
Clyde Phillips is not only the person at the helm of the revival but also the person that made the show what he is. After Season 4, the showrunner left to spend more time with his family and was replaced by Chip Johannessen. When the reception became bad, Scott Buck was hired as a replacement though there was no major improvement.
The decision to bring back Phillips from the revival wasn't surprising since it's under his tenure that the show attracted the most viewers and also impressed critics. The 7th season only had a Rotten Tomatoes score of 33%, compared to the first season's, 82%. Thankfully, the revival has brought viewers back to familiar territory as it currently has a score of 77%.
Community (2009 - 2015)
After Season 3, showrunner Dan Harmon was fired for being difficult to work with. His replacements, David Guarascio and Moses Port didn't do a particularly good job so Harmon was rehired for Season 5.
Soe showrunners put a personal style that's difficult for anyone else to replicate and such was the case with Harmon. According to Rotten Tomatoes, Season 4 is the only one with a critic score of less than 85% (it has 65%). Viewership figures also sunk briefly to 3.58 million during Harmon's absence, signaling general audience dissatisfaction.