As a counterpoint to your claim that being a food source for humans guarantees survival, the orange roughy is a good example. It is a fish that was discovered as tasty and then fished to 10% of its original numbers within 50 years.
These creatures have such a great name! Nobody wants to eat crickets or worms, but who could resist a tree lobster? It is such a great marketing name!
[1] http://www.economist.com/blogs/dailychart/2011/07/global-liv...
Edit: you are right that being a food source for humans doesn't guarantee survival. It only happens if the food source can be mass produced.