But at some point oil will be $200 and then $300 etc. At some time in the future this will become economically viable.
Not really, the problem is right now most plastic ends up burnt up or in landfills, when oil becomes painfully expensive and this becomes an almost economically viable way to "produce" oil, plastics won't be being made anymore as the oil will have made plastic way to expensive to produce and use.
At that point there will only be a very small stockpile of plastic to be converted back so the miniscule amount of oil it produces compared to the tonnes and tonnes of stuff we use a day will be completely and utterly insignificant. Digging up landfills will be a non starter in terms of an attempt to get more plastic to turn back to oil.
RIght now loads of countries are burning up tonnes of plastic every couple minutes to get rid of it.
This is the fundamental problem, right now it costs more than the oil you get is worth and thats before you actually build industrial scale plants to do this, which will cost billions to start up, for a long term loss to be made.
By the time oil costs enough, there simply won't be any plastic being made to then turn into oil, its a complete non starter.
In reality every country in the world should be forced to build these on their own dime, send all plastics there for recycling and eat the costs, but no one will.