There's a reasonable argument that turning them to biochar would enable us to enrinch soil enough to make it globally more efficient at capturing carbon. That

There's a reasonable argument that turning them to biochar would enable us to enrinch soil enough to make it globally more efficient at capturing carbon. That would do both: the carbon in the plastics gets captured, and a carbon capture mechanism gets boosted. The thing you've got to remember about plastics though is that only a relatively small fraction of the carbon involved in producing a lump of plastic is actually in the plastic itself. The production process emits a lot more. If what you end up doing is creating a demand for more raw plastic to be produced, that's a massive net loss.