Of course. Not enough competition, and one ethnic group, who’s factions bring nothing new or special to the table, and all think the same is one way to remain stagnant.
Societies like Japan, and the Jews people love to talk about as having the best outcomes are heavily assimilated. Beyond the huddled masses the West accepts, it attracts the smartest of its human imports by offering them stability, resources, and freedom, in exchange for their mental output.
The southern Somali Bantu, who’s mortality rates, sustainable farming culture, and work ethic is far superior to ours could easily be the group that overtakes Somalis, never mind actual immigrants (which in its current state it holds no attraction for anyway).