Landes & Posner also never advocated abolishing copyrights altogether... They just advocated moving to renewable terms instead of automatic 70 years to life.
Regardless of whether there is a decline in production, there would be a certain decline in production of high-quality, expensive content. If you think expensive to produce != high quality, you're wrong. Expensive content is only produced because it earns more money, and thus is better liked by more people. That's the only objective measure of quality I know.
It might seem very intuitive to say such things, but it is not truth. The truth is that the economic evidence is just not there. What do Landes & Posner say?
"Economic analysis has come up short of providing either theoretical or empirical grounds for assessing the overall effect of intellectual property law on economic welfare." -- 'The economic structure of intellectual property law'; Landes, Posner; 2003. Conclusion, p422, s3.
That means we do not know whether it does good overall. Hard to quite believe? Well there it is.