Who should pay for the content filters?
What if free copyright is only available for five years? After that the copyright owner has to register and pay a fee to maintain copyright (which would go towards public copyright filters) - not a lot to start - say $50 per year per item for years 5-10 - but enough to ensure that only commercially valuable creations retain copyright. After ten years, raise the fee to $1000 per item. And after 20 years raise it to $10,000 per item, all with the copyright holder being able to re-register even if the registration had lapsed.
That way the big commercial successes retain copyright long term, but at a cost (which would reflect the fact that these are profit-seeking adventures), and secondly would ensure copyrighted materials are clearly registered, while low level works quickly enter the public domain. So if you're say a prolific photographer, you have five years to test to see if the photos are worth something, and then you register all those with proven commercial value, with the additional option to re-register (for the going fee) if anything else becomes successful later.