Governments aren't able to change the way bitcoin is used, they aren't able to just print off 1 million bitcoins all of a sudden like they do with their national fiat currencies. As you said, bitcoin is decentralized, and the only way to eliminate/regulate bitcoin usage is if the government managed to shut down all the nodes running bitcoin software/or changed that software into their version. But there will always be people using bitcoin, it's impractical to shut/change everyone.

The way that governments can regulate it currently though is through regulating bitcoin exchanges, by implementing strict AML and KYC laws. As it deals with banks, governments can do something about this. But in the futuree, these sort of regulations won't even matter anymore as bitcoin gains mass adoption, and peoplea rea ble to accept bitcoin without going through an exchange.