There are sea snails living near volcanic vents that metabolize the extra iron into shells and plating. I expect we'll see a similar adaptation at some point. Simple organisms issuing plastic in some sort of process that toughens their shells. If left unchecked we may see an evolutionary leap as more things adapt, and a die off as plastic becomes a finite resource. Future generations may argue the merits of artificially supporting the microplastic ecosystem in the same vein that we argue about reducing it.