You can not have multiculturalism if you want to keep a national culture. You MUST have a national culture if you want to have a nation.
Denmark has one culture. America has multiple cultures, each one indigenous to its region. And new cultures keep coming, and it takes time for them to earn a place in the indigenous culture of that region. But that’s possible only with limited, vetted immigrants, over an extended period of time. At first there’s friction, discrimination, disdain, and slowly, maybe after a generation or two, the newcomers find their place. A pell-mell invasion doesn’t work that way. It’s a hostile takeover, and an intentional erasure of the indigenous cultures of all regions affected.