Ok here the steps:
1. Some government tries to buy up all the Bitcoins, so they pump millions after millions in it

We have two options:
1) We still have enough coins to trade otherwise -> Bitcoin is a success o,O
It's nearly like an official currency of that state now.

2) They have bought all the coins and trading is impossible now (I don't know how they do that, but let's assume they did)
a) We are rich in fiat now
b) We create a new Bitcoin fork (or use Litecoin or whatever)
c) Invest 1%+ of our money in it to have the same Bitcoin economy like now (or more, if we spent more)

-> They have useless Bitcoins bought for millions and we have the same we had before+more