Seasteading is about the frontier. It's not about any political system. Lest we forget, there's quite a tradition of utopian thought within the progressive worldview as well. I think it's wrong to dismiss ambitious idealism as unrealistic.

Worst thing that happens - a bunch of people that don't want to live in the US anymore drown in the ocean. Best thing that happens - they experiment with new forms of government, some of which influence people back on land. Either way, we need more experiments in government, not less.