Earlier quoted context omitted.
They would be hard to establish if you didn't have data on the kinds of health problems that would only show up post-plastic.
Why do you think that? If microplastics is a problem, I would expect studies to have shown up linking microplastics to cancer in the elderly, or to dementia, or heart disease, or any of the other very widely spread health issues that could arise after decades of exposure in a population. I'm not an expert, but wouldn't you need just a scalpel, a microscope and lots of donated tissue samples?
This is because plastics became common when a lot of other industrial practices and materials became common in the environment, many of which we know contribute to diseases of modernity.
Microplastic pollution is likely easily confused with other forms of pollutants that were easier to detect historically, and thus care way more about. This would include lead poisoning or carbon monoxide or other chemicals that mimic hormones in the endocrine system.
So why isn't this an argument for disregarding microplastics as being marginal? Because there are microplastics on the larger end which exhibit effects on the endocrine system more definitively than smaller plastics, and these results are easier to report because these plastics are easier to observe.
But with the presence of smaller plastics everywhere, which we would have to know because larger plastics always break down into smaller ones over time, to both understand that they are a common water pollutant yet also observe them in biological tissue requires more precise modalities than historically have been available when microplastic pollution could have started.
It's possible that it might be only "larger" microplastics that have an effect on health, and smaller ones end up being flushed out before they have an effect, or end up not mimicking any hormone and just end up being an chronic inflammatory like pollen would be to those allergic to it.
But that cutoff is quite arbitrary if we rely on just the microplastics that are easy to observe as the only ones that affect our health. A better idea would be to suspect smaller plastics and then gather evidence on their effect on tissues over other pollutants, as the ability to sample them becomes available.