Question Time: What's wrong with plastic?

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Jack Dellevigne asks:

Plastic is considered bad for the environment but why? Until the ocean dumping saga, which does not have any connection to the US, it was mainly a mantra that plastic fills up our trash storage sites (landfills), but we have plenty of space for landfills and for most of the country that is where our trash goes, and contrary to common belief, nothing decomposes in properly constructed landfills. Like plastics, most everything we dump will stay there forever.

So my question is, since environmental dogma wants to sequester carbon to keep it out of the atmosphere, why isn't plastic waste the ideal way to do that? A pound of polyethylene or polypropylene is 86% carbon. Put it away forever in a big plastic bag, which is what modern landfills are, forever.