The vast majority of content in the BBC archives is not being monetized. Of the content that is being monetized (e.g. Only Fools and Horses) most of it has had long enough to make enough money. Under this proposal, programmes like this could of course still make money by being sold in the shops, but they would make less as they'd be available online for free legally. There may also be some problems to do with programmes that the BBC doesn't own the full rights to. Perhaps the people that own the rest could be compensated if their programme is currently making money, but this doesn't have to be necessary. They were given two benefits by the State when they made their programme: copyright, which, right or wrong, is a monopoly; and they were given help in producing the programme by the BBC which is a publicly funded corporation. The former benefit is something that shouldn't necessarily be set in stone, and the latter is something that other programme makers don't get when they make programmes for a private company.
A point that should be made is that the vast majority of people have already paid for this content. Or our parents and grandparents have. Yes, the BBC should have some time (I say ten years) to try and monetize content, but then let's give the public full access to its programmes after that.
This content is our cultural heritage. Remember, we're not just talking about tv serials here. We're also talking about the Old Grey Whistle Test. We're talking about millions of news bulletins and interviews. We're talking about old sports matches. Some of the most profound, controversial, and eponymous moments in our nation's and our world's history are all there for all to see. Let's open them up.
We live in amazingly creative times. Look at the stuff going on from WIkipedia to Youtube, 4chan to the Blogosphere, look at it and tell me with a straight face that freeing up our content is "moral degeneracy" or some other smear.
Look at someone like Adam Curtis. He has access to the whole of the BBC's archives, and look at the kind of stuff he manages to produce on his blog on the BBC website. That's just one man, blogging on one subject. Imagine what millions of people could do with the same access he has! The next Scorsese or Mozart (or your choice of famous artists) could be a teenager in his bedroom at home, or even a middle-aged person stuck in a rut who turns to the web for his fun. We should free these ordinary people's creative talents by freeing up our content kept under wraps by the BBC. Open it up.