Agreed up to a point. However where there are low food levels in my experience it is the otter that has been the main cause of it...and i have witnessed piles of bones on a regular basis...perch, pike, barbel, chub...particularly when they have a few nearly mature kits.
However, i am also hoping that when they have been here in numbers for a decade or so and they have lost their novelty factor to the general public, it is most likely that their numbers will be allowed to be controlled again...just as badgers and foxes are. So if they dont find their own balance (which i think they will, as you say they are opportunists like most mustelids, not killing machines, the issue being that initially the opportunity was just too good/easy)...we can always find the balance for them! After all, often sense does prevail in the end, and not even the full and completely biased might of the RSPB could protect the cormorant completely...otters dont have an RSPB...thank god.