>And, it further presumes that all the accumulated plastic resides within the gut (it doesn't)
Where does it reside?
Once the pieces get small, they tend to migrate and accumulate throughout an organism. e.g. https://www.acs.org/content/acs/en/pressroom/newsreleases/20...
No one really knows what the impact of this is. Maybe it's not too bad. Maybe it is.
That's a lot of ifs. Right now significant biodegradation of plastic by marine bacteria is still pretty unlikely. Further presuming that the bacteria capable of this will also evolve to exist within the gut of higher marine line is a bit of a leap: guts are a pretty harsh competitive environment and "random" bacteria you ingest don't go live there. And, it further presumes that all the accumulated plastic resides within the gut (it doesn't).