For me Bitcoin will be replace by another cryptocurrency if bitcoin reach the limit of 21 million. But right now bitcoin cant be replacing by another cryptocurrency because bitcoin is the most expensive, popular and leading cryptocurrency of all and many people are accepting, using and willing to try bitcoin. 
If bitcoin could not be replace when the 21 million coin are yet to be mined, it is sure to get even tougher after the 21th millionth coin gets mined, because the scarce nature of the coin would now take center stage and make the coin even more valuable and robustI guess you meant to say bitcoin is the most valuable currency and not the most expensive!