As I have stated before goldfish are self regulating their size, based on the size of the container they are in.
A goldfish will never out grow his environment. Take a small goldfish and put it in a larger container and it will grow larger. The feedback from his waste products limits his growth and prevents reproduction.
I think the same will happen to the human race. Microplastics, chemicals, radiation, war, and as a result of the foregoing, famine.
Eventually the human race will become mostly sterile, unable to reproduce.
This will result in a steady decline of humans to a sustainable level.
Maybe a hundred years, maybe less. There is already a marked decline in new birthrates all over the world. It has dropped below a sustainable rate in some countries, mainly the most highly developed countries have the lowest birthrates.
The plastics and microplastics will eventually sink to the bottom of the ocean and be buried by the constant "snow" of decaying particles from above. I think this will be a marker of the human race for future archaeologists to ponder as the ocean floor is upthrust by tectonic churning and kneading of the thin outer shell of solid matter.
Of all the species that have ever existed on this planet, all of our current living species are only 1 percent.
Pessimists are never disappointed.