Bill Gates buys all the Bitcoin, sells it to the government, taxes everyone, we lose.
Bill Gates would run out of cash pretty quickly as the exchange rate would rise significantly.
But let's say for whatever reason he happened to have been able to buy all 10,948,725 BTC that have been issued today. But in the next 24 hours there will still be 3,600 BTC issued (144 blocks, 25 BTC per block). Then each bitcoin hold 100,000,000 Satoshis.
So each day there are 3,600,000,000 new Satoshis added to the bitcoin monetary base. In two days there are enough Satoshis so that every man, woman and child on the planet could get one and there would be some Satoshis left over.
About the worst outcome from that is the government starts selling these coins and that introduces volatility, but then they would no longer have "all the bitcoins", so that defeats the point you were trying to make, I think.
