Multi-Culturalism is in fact a failure, and was always doomed to failure to begin with.
Culture is a set of norms, that allow different people to interact with each other, in ways that are mutually beneficial, and have a common ground of understanding.
Common culture is what allows two men who have never met each other before, to sit down and talk about American football, and instantly have common ground to communicate.
I have talked about one of my best friends, who was a Muslim from Somalia. Great person. But the truth is, as good a friendship we had, it was often very difficult to have common ground. I would talk about something going on in my life, or a holiday like Easter or such, and they would smile with this glazed look. They were trying to be polite, but they had no idea at all what I was talking about.
Equally they would talk about some Islamic holiday, or Ramadan, and I would smile and be polite, but I had no idea what they were talking about.
Some of their customs to me, were nutz. Surely some of my customs to them, were nutz.
At one point, they were asking me why one of their friends, got a fine from their land lord. As it turned out, their friend had rented an apartment on the second floor of an apartment building. They promptly engaged in a normal cultural custom, of splashing their head, and feet, with water before dinner.... in the middle of the bathroom. The renter in the apartment below, started getting water dripping on their head, and contacted the land lord, who fined the friend.
A perfectly honest cultural norm in Somalia, big problem here in the US. Cultural norms have thousands of effects throughout any society, that can't be easily quantified, or accounted for.
Even the simplest things. I once worked with a guy from Romania, who fled the civil war, back in the mid 90s. He was at Wendy's his first day. The guy sat down and started chewing on his lunch, with his mouth wide open. In Romania this is normal. All the teenagers started making fun of him, he got very angry, as you would expect.
If all the teenagers at Wendy's had been raised under the Romanian culture, they would not have laughed. This would have been normal to them. If he had been raised in American cultural, he would not have eaten with food spilling out of his open mouth. It would have been normal for him to close his mouth while eating.
Multi-culturalism, inherently results in conflict.
When you look at the people groups that have immigrated here to the US, the ones that often do the best, are the ones who adapt to the cultural. Now that doesn't mean they completely discard their own cultural entirely..... it means they adapt.
Those that choose to not adapt, and adhere completely to their cultural of origin, often end up very isolated, impoverished, and usually violent at what they see as slights to their heritage.