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.
I am of the opinion that if 21 million BTC has been fulfilled, there will be no more bitcoin mining, so logically the price of bitcoin will soar because the amount is limited. Something limited will be an attraction besides its value because the amount does not increase.