What if there were a terror attack like 9/11 financed with cryptocurrency or the US government decided that the risks to financial surveillance and tax collection posed by cryptocurrency was so high that it decided to launch a war against cryptocurrency like its war against drugs or radical Islam and was willing to spend upwards of a trillion dollars over the next several years to combat it? Couldn’t they just mount 51% attacks on every current and future cryptocurrency and freeze all transactions?
 
For example, ballpark figures to duplicate current Bitcoin network hashrate would be $600 million equipment cost and $5 million per day electricity cost.
 
Might a response be to create niche, private, “underground” non open source, restricted access cryptocurrencies? Could these gather enough network effect to have any utility without being vulnerable to attack?