It mostly about he control over the macroeconomics which keep those people afraid about the increasing adoption of Bitcoin a and other altcoins. For instance, let us assume that a country has a lot of adoption of Bitcoin and the government wanted to change the number of FIAT in circulation, adjust the interest rates on credit, sell debt, etc. Most of those things would not have the effect they would want it to have because an important percentage of the population would be isolated from whatever the Central Bank or the Treasury of the Stated wanted to do with the National currency.
That alone is enough tho keep bankers up at night.
Your points are valid @Hispo but I think the government is also concerned with other problem. Bitcoin might not be able to hinder the government from engaging in its economic policies to control the economy because the currency has a limited supply and it is global which means that only a few citizens of a particular country might own it. Secondly, the government has not been able to control the influx of other currencies in my country. Currencies like dollars have always been in circulation side by side with our local currency. The government can also limit the circulation of their currency and adjust interest rates because Bitcoin will have little or no impact on their policy. I think their concern is mainly the failure of centralized exchanges and crypto-related firms, tax invasion, and control of the financial transaction of citizens.
Well, you points are valid as well.
But the maximum circulation of Bitcoin in comparison to the population of a country or the world is mostly irrelevant for adoption, because that matters in that sense is the market capitalization, in my humble opinion. Everyone in the world can own Bitcoin because the liquidity will be always be high enough, of course, that scenario would imply a massive increase of the prize.
On your second point about FIAT vs FIAT, what do you think your local government felt when people started to use USA dollars instead your national currency? Most of the times local governments are not happy about it, because they indeed lose control over the macro economics.
It is a fairly similar scenario when we talk about globally recognized assets competing against each other.
If a bank wants to increase the interests to decrease inflation but they only have local currency instead Dollars, then people could still opt to borrow in USD elsewhere. That is why in Argentina the interest in Pesos are huge, but they are low in USD. And there is not much the banks can't do about it.