Plastics are a broad category. But specific plasticizers, like BPA, have been demonstrated to cause specific endocrine issues, up to and including a causal link to certain cancers, miscarriages, and other reproductive/immune issues. And it's not just correlations being found, as the research is showing the mechanism of action by actually inducing the effects in vitro.

And so when a particular plasticizer has been shown to be harmful, the research goes into other chemically similar plasticizers to see whether they have biological effects, as well. BPS is another plasticizer that is being studied, as it is chemically similar to BPA.

So we haven't shown that all microplastics are bad. I'm skeptical that these effects would extend to all plastics. But some common compounds that are present in many plastics are a cause for concern, and the difficulty in treating water or waste for microplastics in general means that some of those harmful compounds are present in lots of places where we'd rather not.

We moved from leaded gasoline to unleaded gasoline based on the specific dangers attributable to lead itself. We can do the same for the specific compounds in our plastics shown to be harmful. Maybe the end result is that we have a lot of safer plastics remaining. But your comment seems to suggest that we not even try.