there's no reason to think there would be no pressure to evolve to eat the monomers once they're there and to adapt the gene for the enzymes from 'professional use' to 'personal use' by the bugs.

I directly address this evolutionary pressure and why there are, in fact, reasons to think it won't behave like lignin digestion in the very comment youre responding to friend.

Lignin digestion is at the end of the day just a random set of mutations that stuck because they were useful. If they weren't useful, to an individual organisms survival, they likely wouldn't stick around, as might be the case with plastic digestion, and would be different fur every single plastic. The same exact method would be used for adding enzymes to their genome in yeasts as you mention or in various organisms for plastic digestion.