Well, at that point, you could use the CO2 to make more plastic rather than releasing it to the atmosphere. My point was, this is how nature recycles biomass every year. Plastics are currently manufactered from natural gas feedstocks; you could make plastic from CO2 just like trees make lignin from C02.

Then, if you buried that plastic (made from atmospheric CO2) in a landfill after use, that would plausibly be removing CO2 from the atmosphere, not adding it. Assuming it doesn't break down in the landfill and leak out as methane or something.