Your contention was that it was an insane position that anyone should be able to use a piece of literature or a song for anything forever without consent. I simply highlighted the absurdity of that based on the fact that:
1. Copyright protections as a concept are an incredibly modern phenomenon, mostly limited in practice to Western Capitalist Democracies. 2. Outside of a short monetisable window (albeit one extended and irrevocably marred by Disney/Sonny Bono) your 'insane' hypothesis is in fact the status quo 3. Much intellectual work is published into the Public Domain, and all copyrighted work eventually ends up in the Public Domain. Your position appears to presuppose a world without such an entity.
As to what copyright actually achieves? It's mostly a mechanism by which the media gatekeepers and owners of capital use legislative and social imbalance of power to deny artist the rights and royalties for mechanical reproduction and otherwise impose financial serfdom.
This is achieved mainly by Copyright Enclosure, whereby musicians are typically pressured or contractually obligated to surrender their master recordings and intellectual property, and by contractual clauses like Controlled Composition Clauses, whereby Labels reduce the mechanical royalties they pay to artists who write their own songs, often paying below the standard statutory rate.