It's likely that nearly free energy is a requirement for mining a landfill so the plastic won't be worth much of anything by then anyway (just make any hydrocarbons you need from the air and water).

Really you'd be cleaning up the landfill more than mining it, but that's okay.

A lot of recycling decisions come down to similar considerations, doing them is a net drain on resources and costs money, so it doesn't make any sense.