In the end, my reasons for moving down the timeline and introducing a new cast have more to do with keeping myself entertained, on the assumption that if I get bored, my readers are going to be even more bored.
People are out of their home on a Saturday night or they're at the movies or they're at dinner and a lot of the people who flip on the television are doing just that. They may have never seen your show before and you can't count on to your audience to be there week in and week out.