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.