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? Couldnt 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?
It wouldn't be far fetched to think that the government could finance an inside job terrorist attack of huge proportions in order to forever deligitimize the usage of bitcoin.
On other thand, a legitimate terrorist attack of huge proportions could be financed used bitcoins (by the terrorists itselfs, so not an inside job)
Both scenarios are possible, and both would be a big blow against bitcoin at a mainstream level most likely..