It's reasonable to assume that if any alternate currency were to really become popular it would get the negative attention of TPTB. Think about it: one day the USG starts to notice an unexplainable drop in revenue. They start digging to find out the cause of this anomaly and find a growing underground economy with a non-USD currency.
For crypto currencies, agencies with lots of computing power would become heavily involved in the effort to try to disrupt or destroy it.
It's what would happen if BTC really became mainstream.
i fully acknowledge that there may at some point be some negative attention given to bitcoin from the US government. i laugh at the idea that at this point, right now, bitcoin is seen as such a threat that billions are invested to "crush it".