I don't understand. The thing they are made from apparently takes hundreds of millions of years to degrade!
I must be missing something important.
1. Don't degrade quickly (per your point)
2. Are difficult to mitigate/filter (per article's point)
3. Are appearing everywhere, including in our water supply, food supply, adult human tissue, and within fetuses still in their mothers' wombs
4. Critically, and probably the point you're missing, they are disruptive to endocrine systems (hormones) which are extremely important to all sorts of developmental, behavioral, and psychological systems, including negative effects on fertility, e.g. a child born to a microplastic-poisoned mother will have reduced sperm count in adulthood. This is a bad thing, and in combination with points 1-3 is potentially catastrophic for human civilization.
Edit, and here: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7068600/
Problem Nr. 2: plastics made from oil are different from, well oil. They're a product, that doesn't occur in nature, as far as I know.
Plastics are hormonally active in the body. There might be thousands of different plastics and nobody really knows how they're reacting with animals, nature and so on.
But the short answer is: petroleum kills, that stuff is dangerous as hell.
How much petroleum does the average human testicle contain?
Uranium isn't a problem if it's 100m underground somewhere in Africa, it's a much bigger problem if you have a refined chunk of it under your pillow