The main thing about plastic is that it’s made from oil, and oil already exists in the ground. Putting it back into the ground is basically neutral minus the pollution involved in manufacturing.

Right, but there's ground and there's ground.

Geological strata vs shallow landfill sitting above aquifers and subject to near-term erosion.

Disposing of this stuff in deep mines seems like it'd be fine, unfortunately we haven't yet, at a society/economy level, found the discipline to do so. Presumably after a few mya of heat and pressure it'll be indistinguishable from other petrochemicals (which aren't particularly nice to begin with).

It doesn't go "back in the ground" though, does it? It gets scattered all over the ecology. When you take something that was buried deep and scatter it all over the surface - especially when that something is oil - that's usually considered an ecological disaster. Deepwater Horizon, the worst oil spill in history, has had catastrophic effects on the local wildlife, and it is still dwarfed in scale by the amount of plastic annually strewn to the four corners of the Earth.