worth of fiat is not dependent on bitcoin's price but on the usage.
It doesn't matter about flat if bitcoin price rises up in the future , if flat is up other flat is up too.
Same thing like cryptocurrency if bitcoin is up , the altcoin would be up too in the future. Yes , as you said is about usage.
Hard to believe that bitcoin price can reach up to $10,000 or even higher .
But once it is happened , it might affected not too much to fiat money . People will just think bitcoin as a commodity to invest instead as digital currency. I myself can not think what will happened if bitcoin price reached $10,000.