I believe this your question is hypothetical in nature because, in reality I don't think that there will be a situation were in a country people will save money in Bitcoin to the point were it will become a problem to the government. I don't see that happening. No every one will adopt crypto that is the fact. Even with the success that crypto currency has been able to record so far, there is still a lot of resentment about Bitcoin from some people.
Bitcoin is made for a selected few , who understand how it works. This is why not everyone will buy into the Idea of Bitcoin and adopt it. Fiat currency will still be in a high supply and circulation to sustain any economy.
Fiat is still the ultimate currency, unless there is an open adoption of Bitcoin and cryptocurrency as a legal tender in such a country.
In cases where the government entertains the idea of Bitcoin and cryptocurrency without fully adopting it, they ensure they make fiscal and monetary policies to ensure they get some returns from Bitcoin investment just as in taxes and it use for remittances in the best use case of transactions.
You are right not everyone would use Bitcoin nor accept its existence, but the few who actually do, are the enlightened ones who are independent and more financially secure with Bitcoin investment as an alternative in the worst financial crisis possible.
