Ethereum will never be Bitcoin and vice versa. Bitcoin was the first cryptocurrency, it is still most popular and recognizable. Ethereum on the other side is widely used, so I see no reason they can switch places. Each one has its own place in the world and in the history.
Maybe it possible to happen to be next bitcoin because it has a lot of a potential like the higher value and and its popularity. But i think bitcoin is still be the number one because it is the most popular cryptocurrency until now.