However bitcoins main reason for success thus far is the fact that it is a challenger to governments and central banks
You are spot on, and everyone, including the governments, knows that for certain.
and can not be shutdown/turned off by anyone government because of its decentralized nature. Governments would have killed bitcoin years ago already if they could do it.
True, Bitcoin cannot be shut down. But, what is it's locus of value? The junction where crypto translates into material goods, whether as barter or via fiat. If you are not using this locus, i.e. if merely hodling, then the value is basically just a figment of your imagination.
And, ..... this locus being a geographically defined spot, falls under the suzerainty of various national governments, all subservient to the major global hegemons, and specifically the USA, if that's your bugbear.
So, why is "big government" merely content to thrust and parry for now, and not make any big moves? It is observing the shape this movement is taking, the way the market is responding. But, make no mistake, it has that critical physical factor of superiority, the ability to impose taxes, and collect them at the point of a gun, if need be. So, it needn't act fast, it can strike at the moment of it's choosing.
What can government do? It can levy a "Carbon Tax" on "environmentally polluting" Bitcoin. It can ban mining on those same grounds like it banned ChloroFluoroCarbons (CFC) a few decades back. It can tax the mere possession of Bitcoin. It can do so with retrospective effect. Say, you held Bitcoin between September 2012 and March 2014, it will tax that amount retrospectively, and God help you if they consider the end-2017 20k peak as the relevant price.
But, you refuse to pay up. They'll lock you up in debtor's prison where they dump all tax evaders. You think of escaping to some sunny island, they'll arm-twist that place's authorities to hand you over. You think you can best them with your libertarian schemes of sea-steads, they'll "send" "rogue elements" to harass your womenfolk and cause you an inordinate amount of trouble.
tl,dr; Long story short, there's no escaping the tentacles of big government. If BTC falls, they'll dismiss it as a "black swan" in their economic record, and no further action needed. If BTC stays put and rises, it'll wield that (super)tax weapon.