A similar attack was made against Wikileaks.
If you can't transfer your cash to Bitcoin, or your Bitcoin to cash, then the decentralized nature of Bitcoin itself is immaterial.
For one, an often overlooked and dead simple way: you can sell goods/services and tell your customers you accept Bitcoin.
You can buy bitcoins for cash from sellers you meet face to face: https://en.bitcoin.it/wiki/Category:Directories
You can mine.
Etc.
You'll have no way to convert electronic dollars into bitcoin. Saying that the government can prevent the conversion of cash into bitcoins is like saying it can prevent the conversion of cash into drugs.
Unless you can turn your bitcoins into goods and services.