1) This can change. Govnts agree on all sorts of things to consolidate power. If the EU/US agree to ban use of bitcoin, then it becomes less valuable elsewhere also.

2) doesn't matter because of 1) - you can spend only on dark markets, and they need to be able to change it for currency they can spend (food etc), so only in countries that allow bitcoin exchange legally.

3) True, but that's not a reason, wiping that out is actually GOOD for fiat currency. Countries ban certain foreign currencies to bolster their own. Banning bitcoin would wipe that and that would be good for fiat. At worse there will be as many winners as losers.

4) Visa is efficient. Its works fine. Bitcoin privacy is better but not good enough. Govnt's aren't not banning Bitcoin because they think its fast and efficient, no minister has ever thought that.

5) So, there's plenty of useful things govnts can invest in - you know roads/rail/hospitals/housing/manufacturing - actual things!

The OP was right to ask this question, and probably knows the answer: A govnt can block its use any time it likes with little consequence to the state. It probably won't though, because Bitcoin is a good Trojan horse, and a spying tool for govnts. It gives them immense control, and the bigger it gets the bigger the Trojan gets , until boom!