So many people miss this point. Most things in economics are positive sum games. But most people treat things like zero sum games (if someone gains something then I have less/am able to get less). But people produce products, people produce value. This is exactly why we have an economy. And it grows (meaning positive sum). We've left so much money and value on the table because of this.
I'd be careful making assumptions about the reasons people don't agree with you.
Taking the grandparent comment: "Supposing this is true, does it matter, if the overall size of the pie increases? Isn't everyone better off? Would you rather the poor stay poor so that the rich don't get richer?"
I don't disagree with this point because I believe economics is a zero some game, I disagree with it because it's wrong (it takes "everyone being better off" for granted), and I will be on guard to oppose any other initiatives (say, open borders) suggested by a group that thinks like this.
