You can argue until your teeth fall out about the benefits or the drawbacks of immigration, but the fact is human nature (which in this case is probably fairly immutable IMO) is to not want a built community to shift violently in less than a single lifetime.

So whether mass immigration is a benefit, is harmful or is just neutral is irrelevant if the follow-on sociocultural effects are those that effectively tear your nation apart by electing a Trump or a le Pen.

Open Borders and pretty neoliberal pseudo-utopian hippie dreams won’t help you then. Not even a little bit.