I wonder if this was around 1990, when PBS lost the edgy stuff and went for bland acceptability and respectability... I think this was a response to the cutting of federal funding, resulting in more wealthy viewer tastes shaping the schedule. We started to see endless reruns of some Peter Paul and Mary reunion concert, opera got bigger, the more challenging drama and other programs went, swear words went.... That's when Dr Who went I know. I stopped caring about PBS then, though Frontline and Nova are valuable, and Bill Moyer's Journal, and Charlie Rose.