The point is that we may not have plastic forever. Oil is a finite resource. An easy, cheap replacement hasn't been found yet. Either we abide by reducing and reusing (where we should be focused), or we should actually recycle.
With our current exposure, it is estimated that 40-50 years worth of oil remains, although there is likely to be new locations found and an overall reduction of oil usage in the coming years.
It will become more expensive when we have to pay for the energy embedded in it, but the difference is not significative on almost any end-product.