It is impossible for two honest, truth-seeking people to disagree about a fact in perpetuity. Eventually, they will realize that either (a) one or another of them is correct or (b) neither is able to convince the other, and both realize that neither has figured out the right answer. This is not an opinion, it is a logical fact. I disagree with other mathematicians about mathematics frequently; it never ends that we "agree to disagree". Either one of us is right, or neither is, and in any case we figure it out and agree by the time we let the issue go.
Many political disagreements are, of course, not factual, but rather based on values. But often what we talk about is facts. So part of my belief that first world countries should accept more immigrants is based on my view that people outside of one's own country have substantial moral worth. But it's also based on my belief that, e.g., U.S. immigrants will not substantially increase the U.S. crime rate, or worsen the U.S. fiscal situation, and will in fact make Americans on net wealthier. These are all factual questions, and I suspect that they drive my disagreement with immigration restrictionists as much, if not more so, than our philosophical disagreements. But the thing is, either me or the people I'm disagreeing with are just wrong, and the fact that neither of us seems to be convinced means that at least one of us is wrong and intellectually dishonest. And I have a hard time believing that I'm never the one being intellectually dishonest.