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. 
It's still not certain, the volume or the number of supply is not going to be the reason why it will be replace, I've seen how people are supporting bitcoin and if you can check on the volume you will see bitcoin is dominating in the crypto market, so it's simply the king of all. This will be replace big holders will dump their bitcoins and go to other crypto currency or altcoins but I don't think it's going to be soon, bitcoin is getting stronger day after day.