Re Doctor Who, the reason so many episodes were lost was not primarily because they needed the old films to record over them. I don't know if that ever happened but at least some of it was just thrown away. That's how some of it was saved, because people just took it home instead of throwing it away. For a long time the BBC didn't air anything more than twice because actors unions objected to rearing recording. Something like 95% of '50s and '60s BBC TV is lost. Doctor Who is just a bigger deal because it ran for decades and became internationally popular. It's actually well preserved for a show from that era because people worked so hard to hunt down lost episodes. Also, there are audio recordings of all of the lost episodes.