I think that Greenberg meant that the Yupik people are so reliant on the salmon as a part of their way of life that the declining salmon populations will lead further hardships for the Yupik in the future. Assuming that my interpretation is correct, it is understandable to make such a conclusion. I think that this is one possible conclusion. While the salmon struggle so do the Yupik, that is true today, but it does not always have to be.
It would be a long hard road, but the Yupik can find a way to survive without the salmon, which should hopefully never be required. It is in human nature to investigate and solve problems, that is how we cover the globe and continue to do so. While some elements of the culture would change, it is possible for the Yupik to continue to record their history and find new ways of moving forward. The fall of the salmon does not necessitate the end of the Yupik people.
Of course, any change in culture would come with hardships. It is not the desire of the Yupik to lose the salmon, to lose a way of life. If all is considered lost as the salmon population declines, then the Yupik people will be lost. Their history and stories, unique only to them would no longer exist. If the Yupik ask “Where do we go from here?” then there will continue to be hope in the future. That future may hold a different way of life, but it would also be theirs to create different from the rest and entirely their own.