The simple fact is that plastics do degrade in the environment, especially in the ocean (and lakes, streams, rivers).
When real scientists went out to investigate the marvelous Pacific Garbage Patch imaginatively described by Charles Moore, they found — well, almost nothing.
I'm a little embarrassed by this since I actually taught a class lesson based on the existence and eco-structure of the Pacific Garbage Patch. That was back when I would assume that the media were telling me facts. Instead, these days I feel we're just to be manipulated by "stories" in service of the media's corporate and political masters.