It's not the mass of plastic, but the surface area. Microplastics have very high surface area to mass ratio, so large numbers of microbes have access to them. I expect to see rotting plastics in my lifetime. But this isn't necessarily a disaster. Many uses (e.g. food packaging) are still possible with biodegradable plastics, and for critical things like medical equipment we have fluorinated plastics, which are very resistant to anything a microbe could do. For everything else we'll just have to revert to wood and metal. This will mean paying more attention to repair and maintenance, and less disposable products.

You don't have to see it, for microbes to find it. We are aware that those tiny spheres of plastic made for facial scrubs, are now everywhere including Antarctica, right? So a microbe that can eat that, will find ready food everywhere, on every continent?