I want to reverse the question and ask; If gold price is $10000, will fiat be worthless? Is there any difference? Moreover when someone bought two pizzas with 10000 Btcs was a clue that btc was worthless? Some people need to understand that fiat is currency and there are strong and weak fiat money. In addition there are strong and weak cryptocurrencies too. So, If people decide that a monetary system is worthless they will ban it from their life. I believe that after the next generation people they use only digital money not because of bitcoin price but because of internet and human needs.
Since gold (or any asset) trades relative to the underlying currency it is valued in, in order for gold to be worth $10,000, the dollar by definition
has to depreciate. When gold drops in price, people generally say
oh that's gold becoming less valuable, but it is also true that it is the dollar becoming more valuable relative to gold. When gold or bitcoin are worth $10,000, whether you see that as the asset becoming more valuable or the currency becoming less valuable, the fiat currency still is not "worthless." As for your last point, it is entirely possible for digital currency to become the norm without cryptocurrency being the norm. Consider, for example, that the majority of the transactions in western economies are already digital, it's just
digital fiat.