Perhaps a naive question, but why is using plastic for energy good?
When we produce plastic we essentially convert oil into a highly stable form instead of burning it into the air. Essentially, we sequester the CO2 that would've been burnt if that oil was used as fuel. That's a huge benefit of using plastics that we are eliminating.
Natural gas can be a byproduct of oil extraction, but it is also available by itself ("dry gas"). IIRC, most natural gas production in the US is of this kind.
Fracked gas in the US is unusually high in ethane, the substance that is the primary feedstock for ethylene (and then polyethylene) production.
In a world without fossil fuel production, reduced carbon compounds will become more valuable. The cost of making plastic may rise, but plastic itself will become more valuable as a commodity for later use (either as a fuel or a feedstock).
PET is not polyethylene. Synthesis of PET involves a more complex feedstock (in particular, para-xylene to make terephthalic acid.) The other component, ethylene glycol, is made from ethylene.
Ethylene from fruit would not be competitive with other non-fossil sources. However, para-xylene can be made from biomass.
And burying that plastic returns some of the carbon to the ground from whence it came. Bury it deep enough and in a few million years it may be cooked, recycled, back into liquid hydrocarbons.
When we produce plastic we essentially convert oil into a highly stable form instead of burning it into the air. Essentially, we sequester the CO2 that would've been burnt if that oil was used as fuel. That's a huge benefit of using plastics that we are eliminating.