I see it possible for both to exist. However, I would be fine with ether or a currency built on ether becoming the main crypto currency . I always say that when bitcoin came out people started to realize the true power of what blockchains could do. It then became apparent that the bitcoin blockchain was not set up to facilitate all of those things. Enter Ethereum... equipped with a built in scripting language to facilitate all of the awesome things that blockchains can do. And yes the block argument is something that i could see killing bitcoin. I like the irony of a community based on consensus algorithms that can't come to a consensus about block size. so at least we have Ethereum!