You appear to be arguing that even if microplastics are present in the environment it’s not a problem. That’s a brave stance to take given the wealth of information to the contrary.

Even if they do break down it’s bad. Plastics have additives in them which are used to improve their material properties. These additives include BPA and PFAS (and similar). We know that these compounds cause problems in humans and the environment. So if they were “easily digestible by the body” that would absolutely be a big problem.

There’s no world in which “nothing will be harmed” by plastic decaying. Some people even argue that conventional plastics are less dangerous in landfill than bioplastics because at least they don’t release dangerous by-products like microplastics or “forever chemicals”.