Who says they have replaced each other. They are just advancement over each other and they have same share (almost) to each other and the popularity too. You can't compare them really as its peoples choice really. Also, bitcoin can be competed by any other asset in the market. Its just about getting popular and letting people know how big profits they can really earn with that asset. Thats the thing people want to choose from the market which can benefit them and if something come up with that idea then sure bitcoin market can go down in no time. :-)