Ethereum will never lose relevance, but the question is asked in a way that it makes you want to say one of them will, you could always say something like both of them will or none of them will. This is why it is important to make sure that you could do as well as you possibly could. This is how you are going to do well, you will end up with a proper return if you think outside of the box, in this case outside of what is asked.

I personally believe that neither will lose relevance, maybe ETH will stay, and the other coins and tokens will stay as well? I think something like matic, ada, sol, avax etc all look like a good thing and should stay, even if a few goes down, something new will come out as well. So that looks like neither will lose relevance in the near future.