When we reach the 21 million BTC then a strong financial system will be developed through blockchain technology and it is not just over then all the transactions will be processed through this where the fiat will nor be independent from crypto and money supply of all the currencies bitcoin holders will have a good reserve of fiat and they have choice of the money whether to use crypto or fiat.
In my opinion, after the supply limit of 21 million bitcoin is fulfilled, the price of bitcoin will be stable because there will be no more mining and hunting by speculators that will spur the growth of bitcoin prices. Bitcoin will become a digital currency that is used globally with a number of changes that have made countries refuse to accept it.