The people who make this argument do not believe in nuance or complexity

They believe that 'more immigrants = more prosperity', full stop.

I'm proof that assertion is wrong. My anecdotal experiences also counter this.

In fact my experience is essentially the opposite. Those that support immigration do it on the basis of proven benefits and successes, and often the conversation is about the nuances of properly integrating them into society to avoid potential negative outcomes.

Many of those that are anti immigration never get to that point of the conversation with us because their views are usually based on a cultural protectionist ideas that rejects the economic benefits. You can't discuss how to properly integrate refugees or immigrants when you can't agree that they should be allowed in. Those cultural protectionists are also largely why societies struggle to integrate immigrants. They become a self fulfilling proficy by creating the very conditions needed to prevent integration and put immigrants, especially second generation immigrants at risk of radicalization or antisocial behaviors.