If most G20 nations ban bitcoins, would that effectively kill bitcoin? If the G20 nations (or most of them) ban bitcoins doesn't this effectively kill bitcoin? My thinking is that a normal person would no longer have any means of converting bitcoins to the local currency. The only other way this would work, were if most places took bitcoin (coffee shops, grocery stores), but if it's banned then most of the places would stop taking it.
My thinking is that (i) G20 nations are democracies, and (ii) Bitcoin is a useful service. If it's useful to people, and people want it, they'll vote for it and it won't get banned. 

If your country bans something useful to the masses, it's not a democracy. It will be banned only because it lost its utility, and if that happens, there will probably be something better replacing it.