Open questions would be: how much use can be replaced with recyclable variants on the application side? (perhaps some use cases are not doable with PET?) How much use can be replaced on the supply side? (can only a fraction of the crude extracted from the ground enter that PET cycle or does a percentage of the extracted mass have to become other kinds of plastics that are inherently unrecyclable?)
And as long as any of those questions stay answered "not all" and we are still burning some of the extracted crude for energy: how much efficiency does that energy conversion lose when the crude takes a detour as un-recyclable plastics application between getting extracted from the ground and ending up in the atmosphere after burning? If that efficiency loss is small, un-recyclable plastics use can be environmentally cheap if it happens within the envelope of unavoidable energy use and vice versa.
(edit: mis-posted before)