When bitcoin price hits 10,000$, Perhaps we will not use traditional currency (cash).
Untill that time, we will be using smart devices to pay everything with bitcoin.
certainly before the price it happen. of course something happens. like all financial systems or digital-based online so demand bitcoin great shot that time, which makes the bitcoin currency that is widely used. and make bitcoin prices to be very high
I don't think bitcoin will make fiat worthless even if it is going to reach $10,00. Because the government will tell if we are going to make crypto currency worthless or not and even fiat.
But for us who uses bitcoin, even they will going to declare that bitcoin is worthless still we are going to use it.
And with bitcoin, I don't really think whatever will be the price of it, it can make fiat worthless.