There is no strong evidence of anything yet, but consider how long it took to "prove" to society's satisfaction that cigarettes are harmful, despite most scientists finding the lung cancer connection quite clear from pretty early. Comprehensive and long term data takes a long time. It could be worth it to be cautious in the meantime.

If microplastics are making our lives 10% worse in some dimension, we will have to stumble onto what that dimension would be basically by luck and then spend at least a decade rigorously studying it before we could make useful assertions.

The hubub about microplastics is that, we don't have great civilization wide health data on most health dimensions, so we don't even have good baselines to figure whether we have regressed in many ways. IF there is a negative effect, it will effect everyone all over the planet and there is no escape

It's an extreme corner of the likelihood/amount of harm graph, and some people think that corner of the graph warrants caution even before harm is proven.

It's the same situation we faced with leaded gasoline, and the US is pretty bad when it comes to those kinds of "mild, diffuse harm that mostly affects people who can't afford whatever system the wealthy use to avoid the harm" problems.

It's not fearmongering, we are literally gambling that microplastics have minimal harm right now.

That is perhaps the most insidious property of all when it comes to microplastics--it's incredibly difficult to work that out.

We don't have control groups, they're found in virtually every complex organism on Earth, including (best we can tell) all humans, so we can't form a control group. We've only recently really started to notice, care, or study them, so we don't have strong historical data to compare against. We don't have many isolated populations (especially of large enough size) where microplastic bioaccumulation is the only major difference in how their lives have changed in biologically relevant ways over the decades, so we can't effectively isolate the effects of microplastics from other confounding factors.

So you have these things that basically became completely ubiquitous--an unavoidable fact of not just human life, but all complex life on Earth--before anyone realized, with several other major global factors shifting concurrently. The end result is that, by the tools and methods with which we perform science, it's nearly impossible to study their exact effects. Maybe they're a slow-burning apocalypse subtly disrupting the mechanisms of life at their most fundamental levels and only getting worse with time, or maybe they do nothing or next to nothing like having a glass of sherry with your Sunday brunch once a week, or maybe they're somewhere in that vast, murky expanse in between the two extremes. Hell, there might even be a net benefit somehow. We just plain don't know, and don't know how we could know, so speculation is just about all we've got at present, and without knowing it's really hard to say if the messaging and literature surrounding the subject is aggressively over-alarmist or recklessly under-alarmist. The best we've really got is the simple fact that we notice them now, and thus have the chance to pay close attention, part of which is regularly taking basic measurements like these to try and correlate trends.

About all we do know is that they weren't here before, and "before" encompasses 99.9999% of all life that we know to have ever existed, so it's definitely weird and maybe probably bad.

There's definitely criticism to be had with the broader state of public health and science communication that harm, or at least the understanding that "we have literally no idea what the broader implications of this are but they're maybe probably not good", are considered to be implicit, either due to fallacious appeal to nature or the simple fact that alarmist headlines catch more attention, generating more traffic and revenue, and thus acceptance rates and grant money downstream. Which is, I think, the real core of the issue.