Before you comment again, you really might want to watch the video I posted a link to.
Plastics don't break down under UV as well or as quickly as most people think. Small particles of plastic are often mistaken for edible food, and since the plastics aren't digestible, often this leads to the death of the animal. Ingested plastics can kill fish, birds, and even larger animals.
In the video, you can see an albatross that died from ingested plastics.
The idea that the open ocean doesn't have a problem is also false... the video features the North Central Pacific gyre, which is an area that concentrates debris and trash. There is also a South Central Pacific gyre. So, even the Pacific, for all its size, isn't immune from pollution.
Some of the plasticizers
act as artificial hormones, and can cause tumors, mutations and changes in the endocrine systems in living organisms.
Unfortunately, your assumptions on the damage plastics can do the natural environment are not based in science, since man doesn't yet know the full effects of many of the chemicals that he has created and made. Many of the chemicals we thought were safe have had unintended consequences, that we were not aware of until much later. DDT, PCBs, dioxins, are all examples of these types of chemicals.
At some point, we may reach a critical threshold and see unexpected and vast environmental changes that will be detrimental to the planet. Unfortunately, mankind doesn't even know if he is on the path to causing such changes or not.