You seem to have a key unquestioned assumption, namely that alternative currencies and their associated economies will never reach a sufficiently large breadth of services to make it possible to do without existing national currencies entirely. I'd question that assumption. I don't think we've gotten close to that yet, but I wouldn't dismiss the possibility of it occurring in the future.
Good point. Any alternative currency must first achieve a level of legitimacy on par with FDIC-insured institutions and governments. By the time they do achieve that, I assume their board of directors will include enough ties to existing economies to make them no more an alternative (as in, capable of defying SOPA) than any existing institutions or governments.
I simply believe this is the only chance to retain freedom of speech. SOPA must be defeated.