[Bjartr]

On the timescales of the biosphere (thousands of years) the vast majority of plastics will break down and hydrocarbons bound in them will be as bioavailable (probably moreso) than they were originally. And on the timescales evolution can operate on, something will figure out how to digest it if there's enough of it available for long enough.

Which has nothing to do with whether or not we should work to solve the harm plastic presents to humanity and existing ecosystems (we should)