I think part of the problem is that people who don't cycle on a regular basis will think "I can get a new full suspension mountain bike with disc brakes for not much more than that from Argos or Sports Direct." They don't appreciate the difference between a BSO and a decent older bike. I bet thouusands of good 70's and 80's bikes with Reynolds frames are chucked on the local tips every year, despite the fact they are perfectly useable and could make really nice bikes again.
I rescued my 1958 Raleigh Trent from a skip and it has turned into one of the nicest riding bikes I have ever experienced. Apart from tyres and cables, the only expense was finding a replacement 4 speed block after I accidentally broke the original.