Ethereum is definitely very interesting technology that should have a nice future if the developers develop everything that are promising. I see Ethereum and Bitcoin very much complementing themselves. They should both have great future.

But if people decide so, they can just ditch Bitcoin and use Ethereum as a currency. I don't see this happening though as long as both chains are robust and work as they supposed to. After all, Bitcoin has great security record going for itself, Ethereum still needs to prove itself.