I think the biggest threat of plastics pollution is loss of our ability to use plastics as a long-term building/manufacturing material. Very few microbes can me

I think the biggest threat of plastics pollution is loss of our ability to use plastics as a long-term building/manufacturing material. Very few microbes can metabolize plastics now, but not zero, and the ability will only spread as plastics (and especially high surface-area to volume ratio microplastics) become more common in the wild. Microbes evolve fast, and they have access to horizontal gene transfer. Heavily fluorinated plastics might survive but I expect all cheap plastics will end up easily biodegradable. It's potentially a major inconvenience, but if we pretend it's as big a threat as CO2 pollution then civilization will have collapsed before it happens, so that's one way to avoid the problem.