There is literally no problem with plastics that stay in landfills forever. Landfills are underground, like the oil that the plastic is derived from.

Landfills are generally very close to the surface, filling holes created by activities like strip mining, so they do occupy surface area. Unless you're wiling to dig down under existing landfill to dig out voids to pack with more landfill.

Landfills are covered when they are full, effectively isolating them from surface ecosystems.

The point is you can't use that area for getting rid of landfill anymore. From that perspective that area is 'used up'.