Look at whats happening in the world right now. Countries are all getting together and making deals to remove peoples financial privacy. Even Switzerland will hand over your details if anyone asks for them. Where there is money, there is always a way. If the governments of the world decide that bitcoins are somehow reducing their taxation revenue they will end them. Its all very well saying it cant happen but people dont put large sums of money into illegal currencies because they have too much to loose. Saying it could never happen is a little naive. Who would have thought you could ring up a Swiss bank and ask for someones entire financial records? A few years ago that would have been unimaginable now its a reality. Where there is money there is a way!
I'm not sure you fully understand how Bitcoin works.. If a government wants to shut down Bitcoin they would have to shut down the entire internet - something that is completely impossible as any person can set up a satellite internet "broadcast". As long as one single person in any country that has somehow managed to disable the entire internet in an attempt to "block" Bitcoin has internet access, the entire blockchain can be propagated to every person in that country and transactions can resume as normal.
The entire Bitcoin economy can function on one single Bitcoin due to its divisibility, so even if governments at some point in the future were to buy up all but one of the 21,000,000 bitcoins that will ever exist, as long as there is one person that retains possession of a single Bitcoin that refuses to sell, they can split that Bitcoin up (and it will now be worth billions / possibly trillions in a fiat-currency sense due to the government buying up every other Bitcoin) and the Bitcoin economy will continue functioning in milli-bitcoins (mBTC) or micro-bitcoins (μBTC).
It's impossible to 'kill' Bitcoin without performing a 51% attack, which is currently impossible, and getting more and more future-proof as more ASICs come online.
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