Humpback whales do have natural living predators - Killer Whales take calves. Since Humpback Whales don't have large birthrates in a female's lifetime. Species such as Humpback whales only need one or two unsuccessful breeding seasons to become threatened or too limited genetically to maintain healthy existence and will become extinct. Them finding 1,000 extra individuals is great, but it's not something that will save them as a species.
Specialist animals such as Polar Bears, Giant Panda, most amphibians (Newts, Frogs/Toads, Salamander), whale species, some shark species, and reptiles will become extinct, because they are too limited climate, diet, etc. Where as the non-specialists are thriving - like most of the Canids and Rodents. (And Humans, by the way).
One of the most detrimental things that can happen to a species though is the infestation of non-native animals. Usually those are non-specialist animals that do not have predators, so they thrive. (Feral Hogs, Red Fox, Giant Snakehead (fish), Burmese Python, Common Carp, Rats/Mice, Cane Toads, and African Honeybees)