That's not relevant. We know it's possible to build a society with no plastics, but the question is how much harm it would do if plastics suddenly started to rapidly decay.

We've only had widespread electronics for a handful of decades, but if we lost it overnight, we'd be unable to produce and distribute food at the necessary scale. Restoring the global economy wouldn't even be on the horizon, we'd be too busy dying of famine.

We've only had the internal combustion engine for a handful of decades, but if we lost it overnight we'd be similarly ruined.

If the bacteria only slowly ate plastics, we wouldn't be doomed, it would just make things more costly, as with way rust and rot.