"When, long before Media7 was conceived, I suggested that back-catalogue episodes of Horizon (which remain very popular on the torrents)"

...and there's the answer right there. When broadcast TV fails to meet the needs of the viewers, they find the content elsewhere. The choices of TV programmers are increasingly out of sync with what people really want to watch - just look at the top torrents, top youtube clips or compare what classifies as (push) TV news compared with reddit (pull). people want science news and cat pictures, not murders and celebrities (although I must admit, "celebrity murders" would probably get me to tune in, out of a grim fascination with the vile depths of broadcast TV) .