Sorting plastic is useful even if most of it is going to be burned.
* Some types of plastic are much easier to recycle than others
* Some types of plastic can't be easily recycled, but are good for downcycling (such as use as filler materials)
* Most types of plastic are fine to burn, but you don't want to burn chlorinated or fluorinated plastic (at least not with very strong pollution controls)
So sorting your waste is good. In our system, we have four types: "hard plastic" (which is probably manually or automated sorted for things that are readily recycleable and to remove PVC, etc); "plastic packaging" (probably burned); "plastic foam" (probably densified and not burned); and "large plastic film" (such as from greenhouses, row covers, construction plastic, etc; I'm not sure what they do with it).
Also:
The low price of wind and solar allow them to be overbuilt while still being affordable - and if you do that, with a mix of wind, solar, and battery storage, you can affordably build a grid that provides e.g. 90%, 95%, 98%, even over 99% of your electricity - but you never get to 100%. You still need some sort of peaking, which needs some sort of bulk storable energy medium. Well, one possibility for that is waste - it's storable and can be burned. Waste / biomass commonly provides a couple percentage of nations' total power needs - which, in a high-renewables grid, may be most or all of your peaking needs.