[XorNot]

Landfilling plastics avoids even putting CO2 back into the environment.

Modern landfill is highly engineered and extremely stable: what goes in there stays there.

Plastic starts as oil in the ground. Replacing it as solids in the ground isn't a problem.

[djentile]

Among other problems, plastics release methane and c02 as they decompose in landfills, so it's not as cut and dried a solution as you imply.

[xnx]

Modern landfills carefully channel and burn off the methane

[ainiriand]

That was always my impression, and after that you can always build a park on top!