> there is a tremendous amount of carbon currently locked up in plastics which is for all practical purposes inert

https://www.eia.gov/energyexplained/oil-and-petroleum-produc...

1.6% of petroleum products consumed in the US went to petrochemical feedstocks (which I'll take to be mostly plastic). Far more petroleum is just burned, and this doesn't even count coal. In terns of CO2, plastics are almost negligible.