There are two different school of thoughts over this topic. One, we people arent accepting change and we dont let system changed. The other one is, time will take all the odds and thrown them out from history. So if we just talk over second one, I think we might be seeing a full felled bitcoin world that wouldnt be involved in dealing of any sort of fiat. Just wait and watch.
The first scenario that you are proposing cannot happen because we are already using bitcoin, so we know at least some people will accept it, the second scenario you are describing will probably not happen either since governments are not going to accept cryptocurrencies while at the same time they stop printing fiat, what we are going to see is what we have right now, a world with fiat and crypto with the difference crypto will become bigger.