E pluribus unum? Out of many, one? It can work, and it did work for a time, although not perfectly. But no longer. Whether a One can be made of Many depends on the nature of the Many.
A viable One cannot be made out of just any Many.
To think otherwise is to succumb to what I call the Integrationist Fantasy. This is the dangerous conceit that people can be brought together peacefully and productively despite deep differences in their languages, religions, cultures, traditions, and values.
To integrate is to bring together into a whole. But a functioning whole, whether political, social, or of any sort, cannot be assembled from any old assortment of parts. In terms of an outworn metaphor from yesteryear, there have to be some constraints on the range of ingredients thrown into the melting pot. Your stew will not be improved by the addition of ground-up spark plugs or enhanced by a liberal dose of WD-40.
Keeping with the gustatory metaphor, wide-open borders is a recipe for disaster.