The longer term, and subtler the effect is, the safer a substance is. My educated guess is that microplastics are going to cause a lot fewer human deaths and ingury than any of the following natural stuff:
Bees, coffee (see acrylamide), peanuts, mosquitoes, arsenic, mushrooms, and bodies of water.
That means all life is going to become more and more exposed to microplastics as more and more breaks down. Sure, maybe it'll be not be so bad for a random human but what happens when this stuff starts getting into single cell and small organisms where it is in far greater concentrations?
What happens when phytoplankton, or zooplankton start getting a lot in them and it negatively impacts them? Do you get a collapse of an ecosystem?
What happens when pollinators like bees start getting large concentrations in them?
What happens as it accumulates in soil to the hundreds-to-thousands of species that are found in the first couple of inches of an average square meter of soil?
And here's the real problem... the plastic is already out there, it will break down into smaller and smaller pieces and if we entirely outlawed plastic today and no more was ever made in human history, the number of microplastic pieces would continue to grow for quite a long time before leveling off and then would appear to start vanishing as the pieces just got small enough to avoid detection via affordable methods.
If you worry too much about what MAY be dangerous, you couldn't leave the bed anymore. Remaining in bed is definitely dangerous. Not drinking water is also definitely dangerous.