"within about 50-100 years"

That's how bottlenecks work, but not evolution. No species is going to evolve in the next century which will be able to suddenly break down plastic and turn it into energy/nutrients.

The ones that do, mostly bacertium and fungi, were pre-disposed to doing so, and didn't suddenly X-men their way into consuming plastic.

As for organisms such as the Chinese mealworm that "eats" plastic bags, jury is still out about whether it's actually chemically converting the plastic or merely biting it into smaller bits.