I would say that there is a lot of obvious evidence for the horrific failures of trying to impose a single culture in a place where multi-culturalism already exists. These failures have been so bad that the idea of cultural purity has earned a bad name.

The argument of what would be better if we started from scratch will hopefully never be relevant since most of us are in nations with multiple cultures.

It's you exaggerating with

> cultural purity

What next no-argument will you show up with, Rasenwahn of the Third Reich or Eugenics?

The OP said that

> the argument for a homogeneous culture is easy to make

and you end the discussion by speaking of the devil and the devil shows up?

How do you create a homogeneous culture out of a heterogeneous one? It's been tried so many times and it has either failed or been judged an atrocity.

Some people want to keep their culture, so you can either force them to change, move them, kill them, or stay multicultural. To avoid being terrible you just have to let others be and trust them even though they are different.

I don't believe I'm exaggerating that cultural homogeneity is a really ugly idea with a terrible history and it is hard to make an argument for it on a national scale.

>How do you create a homogeneous culture out of a heterogeneous one?

The U.S. used to do it to a certain extent. It was a part of the process of becoming American by joining the "melting pot" of cultures and becoming genuinely American. The idea of multiculturalism in America is relatively recent.