Just that the adaptation process might be too slow. I don't think the speed of evolution can keep up with us throwing new kinds of garbage into the ocean.
It's the same with mono-cultural farmlands. By reducing the habitation areas of animals, the population becomes so small that thinking "they just have to adapt" becomes a ridiculous statement.
Sure, but the change will be slow, and humans are fast. By the time that comes through, it won't be of use to us. Even if it were fast, there's no guarantee that it will be just as useful to us after the change, or of any use at all. Just because the oceans will still technically be alive in some sense does not mean that they will be as lively or at all the same as what we have now.
It is hardly even technically correct. Nobody disputes calling the late permian extinction event The Great Dying, even though technically not everything died. Life was disrupted in an unprecedented way, such that people agree this name is fitting. The same is true of a 3° horror scenario.
