I'm sure that Bitcoin will be number one all the time, and the Etherium is number two in the world of crypto currencies. I do not think that other coins can replace them.
of course bitcoin will be the number one coin because bitcoin is the main coin. in my opinion etherum could be replaced because etherum is an altcoin and of course the competition for altcoin will increase. but it is difficult to predict whether there is anything to replace in the future and so on.