Cosmetics were a scapegoat. All plastics breakdown into smaller and smaller particles, but never fully degrade. Ever seen UV damaged plastic that felt like it just turned to dust? That dust is microparticles.
Only if it were that clean: "Finally, waste-to-energy plants have the potential to emit low levels of toxic pollutants such as dioxins, acid gases, and heavy metals." [0]
Just plastic is never "just" plastic. There will always be contamination of trash with potential for emission of pollutants.
Your whataboutism still proves that not all plastics degrade to microplastics, doesn't it?
There is also a difference between the bulk of some material becoming something else and traces of some side product being emitted, not to mention potentially better technologies like molten salt oxidation or plasma gasification.