Immigration when done right, can have a positive impact. Both economically and culturally. However, I think mass migration into declining populations risks having the opposite effect. I’d like to present a historical example.
Hawaii has a famously multicultural society. That being said, it became that way because rich plantation owners simply wanted cheap labor. In the span of a few decades the native Hawaiians became a minority in their own land. Traditional Hawaiian culture is practically extinct as a result. The economic situation of native Hawaiian communities remains impoverished to this day. They wound up being marginalized and almost all of the economic gains that resulted from the mass migration went to those who implemented it. Going further, the Kingdom was eventually overthrown. This was partially enabled by the fact that the new population was so fractured and disunited.
Since statehood, the politics of the state have been largely been controlled by Japanese Americans. Japan was the largest source of immigrants to the Kingdom. There is nothing inherently wrong with this, however. I allege no racism on the part of the descendants of plantation workers. I just wanted to illustrate how a society can be radically altered through mass migration in a relatively short amount of time.
I can only speculate, but I imagine if the Hawaiian people in the 1800’s had known this would happen, they would have chosen a depressed economy instead.