I mean sure, if you’re talking just manipulating some cell mechanisms to produce the enzymes required for digestion like we manipulate yeasts and e. coli to make drugs - the bugs don’t actually use those for anything and they’d lose the trait out in the wild or just keep it as a vestigial mechanism in limited populations.
But I was thinking more in a sense of what happened to lignin digestion. In the end, it’s still a source of carbon that can be used as a building block and the chemical bonds can be broken up for energy, so 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.
Case in point - mushrooms eating fallen logs and strains of S. cerevisiae producing amylase. At some point it made ‘sense’ to just keep those and that gave them an evolutionary edge, so the trait remained. And now we have another pest on our hands - S. cerevisiae var. diastaticus, a pox on non-belgian breweries everywhere. And critters that eat improperly treated wood beams and cause unpleasantness in wood framed houses.