I was once a maximalist, believing that one cryptocurrency would win out above all others. I am still of that belief in a way…
I think what you are failing to see here, is that ethereum is not truly designed to be a currency. Currency is sort of a byproduct. So in that way I believe ethereum and bitcoin will coexist at the top of a crypto-buffet.
Bitcoin is a currency, by design, and proven. Bitcoin will continue to be the place where people choose to store value securely.
Ethereum is the base for applications and smart contracts. It will be functional outside of being strictly a currency. It has the potential to change the way the backbone of the WWW functions.
Therefore, I see the two not fighting each other, but complimenting eachother; coexisting in a peaceful manner to change the way the world uses technology.