They may yet learn to farm nutrition from plastic during a different mass extinction.
1: https://neurosciencenews.com/microplastics-brain-health-2779...
Take lactose tolerance in humans, for instance. That happened almost spontaneously in Northern European populations 6kYa, as it was such an advantageous trait for herdsmen, but then spread slowly - to the extent that it is still not a ubiquitous trait of sapiens. Many such examples, human and otherwise.
I expect there are already plastic fixing bacteria out there, but they have not yet proliferated or optimised the trait. Should other food sources dwindle for whatever reason, this trait will become ubiquitous, rapidly, as competing populations die off.
Human cells can also (mostly) function anaerobically (aka in low-oxygen environments). It’s terribly inefficient though, and we can only do it for limited periods of time before something bad starts to happen to the overall organism.
So we prefer oxygen, and as long as we have oxygen rich environments, we’re never going to get that much better at being anaerobic eh?
One example was of a species of birds in a specific area starting to develop shorter wings with higher maneuverability, because the ones with larger ones were more likely to get hit by vehicles: https://www.npr.org/2013/03/22/175054275/birds-evolve-shorte...