For something completely new like that 50k years is not all that much.
Perhaps enough time for the existing plastic eating bacteria to spread across the world, tho a lot of plastics are buried deep in soil already, so artefacts might survive. Same as with wood, before the wood/cellulose eating bacteria got all over the place, dead trees just for buried. So at high pressure plastic might turn once again into oil.
For something completely new like that 50k years is not all that much.
Perhaps enough time for the existing plastic eating bacteria to spread across the world, tho a lot of plastics are buried deep in soil already, so artefacts might survive. Same as with wood, before the wood/cellulose eating bacteria got all over the place, dead trees just for buried. So at high pressure plastic might turn once again into oil.