I suppose anything could be used as a currency, even if that was not the intention. Like how prisoners trade cans of fish or cigarrettes as currency. If Ethereum has the majority of the market share, then it COULD be the de facto curency.
Realistically, I'm thinking that Bitcoin will reign as the top internet currency in the future.
I remember reading about that maybe last year? Cigarettes are now the second most valuable currency in prison. Ramen noodles are now top, and especially in USA and Europe actually! ETH may never reach de facto of Bitcoin but it could have its own niche, the way Monero has a niche. ETH the investment currency maybe.