“Costs” often ignore externalities like environmental damage and inequality. Landfilling or dumping plastic may be cheaper now, but it shifts the true cost — centuries of pollution —onto vulnerable communities today. There is a reason the clothing dumps are in Ghana and Chile, rather than wealthier nations like the US or Germany.

If the price to companies profiting from plastics included exteralities I could possibly agree with you but as it stands these costs are normally paid by disadvantaged individuals or marginalized ecosystems.