That fact that animals end up with plastic in their digestive systems is quite uncontroversially a terrible thing, wouldn't you agree? I can't elaborate on the potential biodiversity collapse, as I didn't make that comment.

Edit: Clearly I just gave the easiest example, not the most relevant one. A basic Google search points to [1], for instance, where you find observations such as

If algae and plankton communities are threatened, the entire food web may change. Animals that feed on algae and plankton, such as fish and turtles, will have less food. If populations of those animals decrease, there will be less food for apex predators such as tuna, sharks, and whales. Eventually, seafood becomes less available and more expensive for people.

These dangers are compounded by the fact that plastics both leach out and absorb harmful pollutants. As plastics break down through photodegradation, they leach out colorants and chemicals, such as bisphenol A (BPA), that have been linked to environmental and health problems. Conversely, plastics can also absorb pollutants, such as PCBs, from the seawater. These chemicals can then enter the food chain when consumed by marine life.

So, I frankly haven't carefully looked into the prospects of "biodiversity collapse", but even that stark term sounds justified. Surely another terrible thing, in any event.

[1] https://www.nationalgeographic.org/encyclopedia/great-pacifi...