Hopeful. But it doesn't take centuries for plastic to break down in the oceans, right? I once heard that if we would prevent any plastic from entering the oceans, they'd be clean again within 10-15 years.
Not sure at all about this. The infamous Friendly Floatees show that apparently fragile plastic objects can survive years in marine environment without great damage.
https://fr.wikipedia.org/wiki/Friendly_FloateesWe have no clear assessment of the speed of degradation in smaller chunks, down to nanoplastics found in food chains.
Plastic, somehow, is akin to wood. Wood can degrade soon in good conditions, say tropical forest, where mushrooms and other agents are pretty much efficient at breaking the carbon chains. In other conditions, e.g. underwater in lakes, or at the opposite very dry conditions, it can survive thousands of years. Even if opportunist bacteria or mushrooms find the way to breakdown plastic in the wild, this will be more or less efficient depending on many parameters. Like wood, plastic comes in a huge variety of composition and structure, and some will be more resistant than others. Devil is in the details.