It will not be instantly worthless. As I keep saying, it will be exactly like the centralized currency of a Federal Reserve, and thus a currency in decline. The driving incentive will be to squeeze and steal as much value out of an economy using your 51% controlling stake, until something comes along to replace you. You can make way more by controlling the currency and squeezing the remaining 49% out of the economy, than by selling 2% of it. And if you don't do it, someone else will. That's the endgame in that system.
Okay, based on this post it seems you have developed some understanding. Greed leads to monopoly because monopoly is always the most profitable option.
The government has three choices:
1) Prohibit Bitcoin
2) Extract Fees From Bitcoin; Regulate Bitcoin
3) Let People freely use Bitcoin without interference.
We can see that, provided the government is hungry for revenue and control, 2 will seem like the best option. 1 will not generate revenue and will cause financial instability. The ECB seeks to avoid this first and foremost. 3 will result in a loss of revenue and control over the monetary system. The ECB seeks to avoid this also. 2 provides control, revenue, and stability. All stuff that governments and central banks like. I understand that you do not like these things and prefer 3. That is called wishful thinking.
What can be done to prevent 2? Nothing very effective, but there are some counter measures. PoS helps to insure users against 2 (i.e. users get a payout from the state if 2 occurs). PoS also makes 2 an order of magnitude more expensive to achieve.