You don't have to wait for such an extreme event to happen to see some impact or disruption of the status quo. After all, it is simply wishful thinking to consider it possible that the majority of the world's population will suddenly convert all their fiat into Bitcoin. That's not going to happen. But the disruption is there. Remittances in some countries, for example, are done in Bitcoin. Some people have also quit saving in fiat and shifted to Bitcoin instead. I mean, there are small yet significant disruptions happening right now. There's no need to see shocking events just to see the impact of Bitcoin.
You need a big event to happen if you want every person in the world to convert to Bitcoin instantly. We would probably need fiat to collapse and that probably will not happen for a long time. Every country wants to keep fiat alive because they profit huge margins from keeping it alive and Bitcoin is a threat to their profits because they can not control it like they do with fiat. They will try to rescue it if it is at threat of being replaced by Bitcoin and I think it might be bans on Bitcoin if we did come close to threatening them because they want people to think that fiat is the only option for them.
If Bitcoin grows steady and people do not jump ship instantly the government might not ban it or put too many restrictions on it. Then when we have enough passionate users who are willing to fight to keep Bitcoin alive we might be in a good position to challenge the fiat currencies but we are a long way off until we can challenge it.