I don't think it's true that plastic is cheap as a function of capitalism, private land ownership, or the general way the world is organized right now. It's cheap because it can be produced quickly and at scale from widely available raw materials. Abolishing capitalism wouldn't change the physics and engineering of natural gas wells, nor of the lumber mills and mining required to make paper and wood.

You mention asbestos, but I'm pretty confident we would not have banned asbestos if we didn't have other comparably effective building insulation materials. (Indeed, it's still used in many poorer countries where people are unable to afford the alternatives.)