How to prevent falling like the Roman Empire (it's politically incorrect)

Ancient Rome fell in Ad 476 when massive immigration lost control of its borders, rather than from barbarian warrior invaders. Before that Rome controlled immigration letting in fixed amounts of people, and telling them where they could settle. It looks like the U. S. will not control its borders either, and we really don't want to become a police state to do so. This means we will have no choice, a wide variety of people, and ethnic cultures, will enter the nation, even though many recent immigrants have not been assimilated.  Then the founding culture changes, laws change, cultural wars naturally take place. This has been happening in Europe too.

Short-sighted and greedy businessmen are mainly responsible for our open borders, they seek cheaper and cheaper labor costs, and they largely control the government and elected officials. Then other individuals and groups ride the tide of open immigration with their own schemes and wills to power.

The hard fact is, in modern times, assimilation has not worked, when ethnic cultures are too distinct from one another they simply don't assimilate, the melting pot hasn't melted. Assimilation only really works when groups are fairly closely related, and even then unassimilated pockets remain. The question not asked is, why should very different people melt into one people, even if they could, which they can't, when that motley condition largely led to the fall of Rome, as well as other great civilizations?

Otherwise it looks like we will fall as the Roman Empire fell. Ironically, we will then most likely split into ethnostates---because real biological grounded human nature works that way. Throughout human history, right up to the present time, human nature has been kin-centered, ethnocentric, even xenophobic, among many other traditional things, with group-selection as the primary unit of selection.  Homogenization and assimilation won't happen unless we genetically change human nature, which is unlikely, since the modern liberals don't like to think about biology or genetics, even though changing human nature would be the only way their quixotic, egalitarian, totalitarian, we-are-all-the-same utopias might have a limited chance of being realized.