If you were just talking about plastic packaging you have a point. By weight that is a small percentage of plastics though, which is incidentally why it is so cheap.

For other applications plastics are an outright miracle material though. Imagine if all drinks came in glass bottles; all cases and other structural parts were made of metal or wood; all elastic members were made of rubber or treated wood. The added need for shipping alone would probably outweigh the CO2 impact from current plastics production. As it is you get plastics almost for free while refining oil. Not to mention the insane demand there would be for metals, wood and rubber.