I think it would be ban of exchanges in the American soil...that can create a very dramatic and possibly crippling effect on Bitcoin though it does not mean that Bitcoin will die. However, if things would be the same as they are now, there is no reason why USA will be banning exchanges considering that in my opinion this kind of business is now contributing to the tax coffers of the government. It would be like shooting your own feet.
Not sure US or Japan would go back on their existing deals with regulated exchanges. Coinbase was perhaps the very early giant to get on board with SEC, while its backers (Bitcoin Foundation) enjoys strong ties with the political elite. See what happened what IRS tried to make a move on coinbase:
https://www.cryptocoinsnews.com/republicans-slap-irs-coinbase-bitcoin-tax-huntAny closure of that alone would result in long-lasting repercussions. Their investors would not take too kindly to that kind of move. NYSE, BBVA for example. And we all know these Americans secretly want to make money with Bitcoin (cough, Dimongate, cough). Like you said, would be shooting your own feet.