I'd argue that if we are burning plastics we get to dodge microplastics, and we get a second use of hydrocarbons. Burn enough plastic, maybe we can displace some gas. Optimistic, I realise.

One problem is that plastic generally contains all manner of additives which create a health hazard when burned. Incinerators supposedly filter out the dangerous pollutants, but it seems likely that quite a lot gets into the air. For some of these chemicals we know that tiny concentrations, much smaller than previously thought, are enough to be dangerous. The safest approach IMO is ground sequestration. And of course much better regulation of plastic additives.