A predator might not need to eat itself out of house and home in order to impact disastrously on a given water. Take the case of a small tributary of a large river. In the large river, which is relatively "healthy" the scale of things can allow for predator/prey to regulate and stabilise and for fish losses to be absorbed. The small tributary is borderline in status, with just pockets of fish as it struggles to recover from its problematic history but often lapses back. Otters from the main river can explore and hunt along the tributary, visiting temporarily or occasionally, creating a new and possibly critical adverse effect on the few fish currently thriving. That's the situation as it appeared to me yesterday when I fished a local small river. Under good conditions, in several swims where chub are reliably found - they don't have much choice about where to be, given the generally shallow water with occasional pools - not a bite. Locals passing by were keen to tell me about the surprising new sightings of otters on the river. I don't see why I would be making a mistake in judging that attention from otters is something this little river can ill afford, and an otter can do plenty of damage here without eating itself out of house and home - it can simply go elsewhere, or return to the main river, leaving the situation much worse than before it came.
More broadly, I'm not altogether convinced that the argument that predators naturally self-regulate in predator/prey balance answers all the issues in play. In other areas of wildlife, for instance. A few years ago, in an area where, for all the mature gardens and apparently suitable habitat, garden birds are scarce, we started to see huge gangs of young and "teenage" magpies. Eggs and nestlings came under unprecedented pressure, and the garden bird population, several years on, is still minimal. It may recover in time, but we live, and fish, in the here-and-now. When we had the warm winters of the mid and late 90's, young swan survival rates went up, and we had big groups of juvenile swans completely denuding a small local river for vegetation, until they moved on to look for something else. When we have so many areas where fish are apparently struggling to thrive, you don't have to be ignorant about the relations between predators and prey to have legitimate concerns about the arrival on the scene of new and possibly short-lived and opportunistic predators whose effect in particular and specific instances cannot be written off as a sign of how our rivers are improving.
Predators and prey may naturally self-regulate in textbooks; but our reality, with all its man-made influences, historic and present, is no textbook. I'm conscious we need to try and understand the bigger picture. I'm very happy to live in a city where peregrine falcons live off city-centre pigeons. Seems to be self-regulating nicely. Equally, I'm very unhappy that a barely viable small river is now being visited by otters. Not all environmental problems can be solved by "leaving it to nature", and not all interventions are misguided.