Landfill as an imagined route to sequestration is easily seen through with the right thought experiment.

If you incinerate the trash instead of burying it, carbon does indeed get released.

If you burned enough coal/naturalgas/oil to produce the same amount of energy produced by incinerating, you would release a comparable amount of carbon.

Burying it doesn't sequester anything, if you assume it gets burnt to produce energy. If you burn it, you displace burning fossil fuels.

I wish recycling was more realistic, it frustrates me to no end that out of all those cute numbers that indicate you should feel good putting it in the recycling bin, odds are that anything you put in made of 3, 4, 5 (sort of), 6, or 7 is counterproductive. It just contaminates the good stuff.

But tangent aside, if you consider the plastics to be fuel it makes absolutely no sense to sequester them instead of burning them to displace fossil fuel use, reduce land use, and also centralize a major pollution source (plastics and such contaminating groundwater near landfills over centuries vs a centralized emissions filter).

The fly in the ointment of this idea that it’s better to just burn it for energy is that 40% of US electricity is already carbonfree, ie not coal, oil, or gas, plus burning trash tends to be less efficient than high efficiency combined cycle natural gas plants.

I do sure wish it was further along, and agree that it makes more sense to sequester it or convert to syngas for converting it back to useful products with more free energy.

It seems like a lot is simple once most electricity has a low marginal cost (environmentally, of course they'd usually negotiate feed-in tariffs or other guaranteed long term pricing that accounts for the capital costs of construction).