bitcoin is not used as a currency, it seems that the influence of total supply is very limited and the price is very expensive so it makes the price of bitcoin very expensive and is not suitable if used as a currency because it will be difficult to use for transactions.
But it has been used as currency for years, people buy and sell things using bitcoin as a medium of exchange, it's a daily routine here in this forum and many others out there do it. What makes bitcoin isn't so good as fiat money due to its volatility, early adopters hoarding bitcoin while the whales on the wall street manipulate the price according to their need to gain profit in this market. The limited supply of bitcoin isn't the main problem, scarcity makes bitcoin won't lose its value to zero instead keep rising.