I think it's a combination. I can pull up evidence... Like studies from ivy league schools that show how diversity leads to bad outcomes generally.

I think the problem is that immigration is being used almost like a crutch to avoid dealing with our own domestic issues. Instead of trying to train better skilled individuals here? Let's just immigrate more. Women aren't choosing to have children? Let's just immigrate more. Company might need to raise wages? Let's just immigrate more. People want vote conservative? Let's just immigrate more.

Also, the way in which one handles immigration is important. When you actually do try to assimilate and have an overall arching single culture, it generally works. When you attack the overall overarching culture and try to treat all cultures as equal, I'm not sure it works. India is diverse but they do have an overarching Hindu culture so in that regard they aren't diverse. Western culture is all about attacking its traditional culture.

While in theory immigration can work and isn't a bad thing. The current situation in a lot of western countries, especially NA is one where no immigration until we get our shit together the dust settles for a bit would probably be good policy.