Sure, the thing that you're missing is that there's extremely strong evidence that these plastics:

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.