> It's one of those environmental slam dunks with zero downsides

In relation to directly burning oil for fuel, yeah. In relation to other disposal methods, there's still the pretty major downside of being dependent on a non-renewable resource, in addition to…

> (Before you ask: Modern incinerators do not release any toxins from burning plastic, none.)

Greenhouse gas emissions are still an issue, though, no? Or do the incinerators capture that?

If you were going to burn oil for power, and instead you burn used plastic that for power, greenhouse gas emissions from the burning are roughly similar. However, you skip the emissions from oil extraction and transport, assuming the plastic is burned close to its use / collection.