Originally Posted by Sly

When characters separate, the show can no longer be about the characters as a group, so the show tries to make each character interesting in itself?

In this case? We get short stories about Don, Betty, Sally, Peggy, Pete, Joan, Roger? None of the stories are big enough to really grab you, and you stop caring about all of the characters because you no longer see their life every episode.

Same thing happened in Weeds, happened in Breaking Bad until they brought Walter and Jesse back together, and an endless list of others. Whenever shows separate characters, they jump the shark.