I used to think burying plastic in landfills for carbon sequestration was a good idea. Then I found out that making plastic releases an enormous amount of CO2.

Thus, successful recycling of plastic would mean less new plastic being made, and less emission. Before renewables, petroleum not made into plastic would instead be burned as fuel. But now, or soon, maybe it will be left in the ground, instead, as not worth pumping out. It is better to leave carbon sequestered where it is than to extract and then sequester it.