I don't think bitcoin will ever be used in that way, it can either replace a currency in a country entirely in which case the price fluctuations would only effect transactions outside their country, or it can be used in such a way where it is converted to fiat immediately at the time of the transaction.
You are far too fiat centric, which to a certain extent is understandable right now, but there might at some point be a time where people don't calculate in fiat terms anymore, but just in satoshis. At that point fiat will be tied to Bitcoin, and not vice versa. Bitcoin is a completely new asset class with the potential of surpassing the current major traditional assets. And yes, it may never turn out to happen, but at least it's something that I will be happily looking forward to. Bitcoin for me isn't just a an investment tool, it's a whole new road to freedom.