Comparing BTC which is equal to 10000 USD, USD will be worthless. But still people will be using USD, unless and until BTC is accepted globally or atleast oil giant countries accept BTC for their petrol.
Fiat does not have to lose all of its current value for Bitcoin to spike, its the same as why oil can be so low and the currencies of the world have no had a major spike in value. It was just that the demand for oil was filled, and there was an excess available.
Bitcoin can easily spike to $10,000 or more if there are enough people that want to buy into it. This can be the overall increase in Bitcoin's value, this can be because people have read something on a large and reputable news source that is telling them to buy into Bitcoin.
As long as there is a demand for Bitcoin, there will be value for it.