meaning that bitcoin is no longer there to be mined, and when the supply of 21 million has reached the limit, then what can be traded in the market is bitcoin owned between users. for example user A, sell and buy by user B. there is no more stock, and everyone who holds bitcoin is those who can still trade bitcoin in the market.
Then maybe we could only trade bitcoin, and maybe bitcoin will still become superior to the other. Still, I don't imagine too far because we still have a long journey and it's not good if we imagine how it will happen. There are so many possibilities for bitcoin to happen in the future since we still following bitcoin journey. The important thing right now is only preparing for the future, so when it happens, we are ready to face it.