I'd have to disagree about 'The Carol Burnett Show'; that one lasted at least a year too long. Harvey Korman was absent for the final season, and they tried to replace him with Dick Van Dyke, who, to be brutally honest, hasn't been funny since his sitcom ended. Van Dyke didn't even last the entire season, and they didn't bother to replace him, but used guest stars Ken Berry and Steve Lwrence more frequently.
Carol's final episode is on YouTube, and it shows that Korman didn't appear with Carol in the new segments, although obviously, he was in plenty of old clips.