If we are going to phase out fossils, we might just as well as phase out plastics last. (By burning it.) It the meantime, figure out how to make plastic less toxic and also how to use less of it.

Maybe in a few generations there'll be a ban on burning any fossil fuels other than plastic.

The energy produced from burning it would come from somewhere else, which on average won't be 0 CO2 either.

It may be

That assumes that it does not decay in the landfill. Even if that is true today, I don't think we can rule out the evolution of bacteria etc. (possibly bio-engineered) to feed on plastics.