I'd say when many of the supporting characters started being used less frequently. In season 33 it became really uncommon for many minor characters to make appearances, but that wasn't the season that put an end to the show having a big cast. I remember catching the show a few tiems in season 29, and then watching most of season 30, and there were a lot of characters who started being shown less, whetehr in old or new segments, who I did see on the show in season 29. There was even a short period when PBS (or at least my local PBS station) showed season 29 episodes again (during pledge season), and near the end of the season the epsiodes where Slimey went to the moon were shown again, and I saw most of the characters who hadn't been seen much (or at all) in season 30. Characters like Guy Smiley, Simon Soundman, Forgetful Jones, Lefty, Sherlock Hemlock, and Don Music. Kermit even started appearing less, and this was two years before Henson sold the Sesame Street Muppets to Sesame Workshop (episodes featuring old Kermit sketches in season 30 were few and far between).