Environmentally speaking, shipping it off to a landfill would have been orders of magnitude better; burning it released thousands or millions of times more pollution. Most polyester resins are aromatic, so incomplete combustion can produce a wide variety of quite toxic substances.

I guess we did release some. Mostly soot and half-burned hydrocarbons to be decomposed by solar UV. Still, thinking of all this just being buried for like 2e6 years ... that seems even more wrong.