For one thing I said there are good reasons to completely ban cryptocurrencies. For another I didn't even say there aren't any good reasons against it.

Your reasoning is faulty, because you underestimate the ability of countries to cooperate. The US could probably even enact a virtually global ban on its own. Yes, some countries could try and resist, or some companies (and criminal organizations, of course) but they would be instantly limited to a much smaller market.

The only thing nations would have to do is make the interface between the Bitcoin economy and the rest of the economy illegal and too cumbersome. Bitcoin wouldn't die, but it would crash hard.