Most consumer plastics are recycleable. If everyone recycled recycleable plastics it would live up to its intended potential. That aside plastics are actually not bad for landfills. Plastics don't biodegrade rapidly or give off the rate of methane release as other garbage if it is buried (not exposed to sunlight). Sunlight speeds up the deterioration of plastic and releases more greenhouse gases. Since plastics don't degrade rapidly, the filled land doesn't calapse or create sink holes due to deterioration like other garbage.

Try this... Dig two holes in your backyard, fill one halfway up with paper garbage and the other with crushed plastic water bottles. Pack both holes firmly with dirt (mound the dirt slightly for settling) and watch it over a few years. After a while the paper garbage side will sink in considerably. After the experiment you can dig up the plastic, wash it and recycle it. Just don't throw it in a river or ocean.