By that line of reasoning we don't need to to worry about steelhead because we know resident rainbows will go to sea. Killer Whales have segregated themselves into populations (which some consider fully separate species) that are feeding specialists. The Transients (mammal eaters) don't switch over. Why would they as there are plenty of marine mammals. I am unaware of any generalist KW and also unaware of any that switch from one food source to another. Maybe they will but probably they won't.

The argument of "something else will full the niche" is a good argument for tossing ESA. Who cares what we lose; something else will evolve and move in.