The price of something usually reflects the energy needed to produce it. But it does not take account the externalized cost on the society and environment.
On the other hand, there are areas where other materials like cotton are only a bit more expensive but do not have such a negative effect on the environment. In those areas, like textiles, a ban on synthetic material is probably a good thing.
The problem with the environmental impact of plastics is not the production, it's where it ends up. So the question is how to incentivize keeping a closed system where plastics ends up being burned.
Minimizing the use of plastics requires a complete change of how the economy works. It would require a society of citizens, not consumers who throw away things that are still fine and there are certain areas where plastics can not be replaced by other stuff.
Forcing a cap on plastic production means hurting the emerging markets most but also the wealth in western countries.