While polymers do indeed photodegrade, that doesn't remove it from the environment. The big pieces simply become smaller. This is mostly a minor irritant on land (where microscopic polymers will just become another inorganic component of soil), but poses serious risks to marine life, who can't distinguish indigestible polymer particles from food.

Thus marine organisms starve to death with guts full of styrofoam and PVC. This is bad enough when the creatures eating themselves to death are cuddly critters like sea turtles or pelicans, but as polymers degrade into microscopic bits the effect moves down the food chain, such that these particles are now replacing plankton. You can knock off the top of the food pyramid (turtles and pelicans) with little effect, but let's think really hard about what happens when you kick out the base.

All of which leaves aside the aesthetic issue of areas of ocean the size of continents that consist almost entirely of rafts of plastic garbage.

Polymers will exit the ecosphere when some organism develops the ability to consume it, or if it is removed geologically (through subduction, for example). Burning plastic will break down some of those long polymer chains into more easily-digestible molecules, but release plenty of carbon and molecular goodies like colorants or other chemicals. Garbage bags -- or rather, the indestructible, mildly poisonous molecules in them -- will be around a lot longer than 500 years.

Orion Online has an excellent overview.