Evolution, like geology, tends to happen in fits and starts - yes, it happens over very long timescales as a general phenomenon, but significant changes tend to happen in very short timescales, before then annealing over a population and finding equilibrium over a longer timescale.

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.