Actually, the government doesn't need to worry that FIAT will be replaced by bitcoin, because that will take decades more. because not many people understand bitcoin. Bitcoin is the technology of the future, so at least by accepting bitcoin in a country we are preparing for the coming crypto era.
You're right. Bitcoin won't replace fiats, Bitcoin only complete the financial system in the world. When we have Bitcoin as the payment tool, we have another option that may ease our transaction on buying for goods and services. We also can transfer money easier without the need of the third parties. However, if people still want to use fiats in their life, they still can do it. I'm sure the government doesn't want to remove their fiats from the financial system. It is simply a bit impossible removing fiats because the government can control the financial system with their fiats.
As a sovereign country, they are certain to ensure that they will have their own Fiat so that there is recognition, because, after all, Fiat is still needed for them for transactions that have often been done, especially for the transactions of traditional markets that prioritize Fiat for each transaction.
Agree with you, that Bitcoin is also important to be able to complement each other in every transaction that FIAT may not be done, especially rapid transactions without limits that can only be done by Bitcoin. So, with the presence of Bitcoin, there are options of choice that can be done besides Fiat; let them complement each other and require and not just the opposite.