What if the government of a country is irresponsible and issues a large amount of currency to buy Bitcoin?
No matter what the reason is, inflation is already very serious now. If some unscrupulous governments print a lot of money in order to repay debts and fight inflation, of course it will not be one time. Credit money no longer has credit.
When a chain of interests is formed between big countries, behind the crazy printing of money is the harvesting of countries that are not rich. What would it look like if it were all exchanged for Bitcoin?
To find the answer to your question we only need to look at the past, what has happened when a country behaves in an irresponsible way and they print a lot of money to buy what they want or to pay their debts?
And the answer is clear, other countries stop accepting their currency as it is not worth the paper in which it was printed, besides even if during such circumstances there will be people willing to sell their bitcoin for useless fiat a great deal of long term holders have been preparing themselves for that scenario, and they will do what they know how to do best, which is to hold so no country should be able to abuse their power and get away with it so easily.