If we're succeeding in making ourselves rational, this---one would think--would lead to a political convergence.

Politics includes much which is a matter of preference, not just accurate beliefs about the world. For example "I like it when I get more money when X is done" is the core of many political issues. Perhaps more importantly, different preferences with respect to aggregation of human experiences can lead to genuine disagreement about political policy even among altruists. For example, an altruist who had values similar to those that Robin Hanson blogs about will inevitably have a political disagreement with me no matter how rational we both are.