Diversity isn't the problem and it's arguably a top 5 reason the country became what it became. The problem with diversity is that if enough people stop believing in the potential of the country and look at it as a quick lick to siphon revenue out of the country, it can become a weakness, because there's no incentive to preserve the nation.
The closest parallel is obviously the Roman empire. While they were diverse, internal conflict lead to 100 years of political strife; crisis of the third century, year with 5 emporors. Eventually the empire was too big to maintain politically and people had their own agendas that weren't in line with preservation of the empire; see the absurdity of the tetrarchy. Each side had their own loyalties and militaries and eventually the regional tribes looked at the empire as a quick lick to make some money and didn't care about preserving the empire.
We are creeping towards that. Corporations off-shoring jobs and revenue, foreign nationals buying property in mass, backwards ass immigration policy that doesn't incentivize anyone to stay or build here, and unwillingness to work together politically. Country isn't going to fall anytime soon, but by the second half of the century we could be in some trouble.