I'd happily pay to get access to the full BBC catalogue - in fact I'd probably find more to watch there than I do on Netflix & Amazon (which I already subscribe to).
Lots of people say that. I imagine the BBC use data from things like https://www.bbc.co.uk/iplayer/categories/archive/ to demonstrate that most people are not viewing the archive content that is available already, so people's claims that they'd pay are not backed up by evidence.
I wonder if they collect statistics of people who spot that there are some old 'Horizons' on iPlayer and then get seriously disappointed at the meagre selection available.