You really think it can't be devalued by the government?
Its value changed when China implemented laws regulating it (and even when there was speculation of it).
If the NSA threw its resources at mining, it could become rich and then dump for cheap, thus crashing it.
The FBI already showed that they can seize some, and they managed to seize such a significant portion that it could be devalued that way.
What you should be saying is "It's not backed by a central authority we have to trust". A third party as powerful as the government can still do significant damage to it.