Here's what Stephen Tual has to say about it:
"Bright. Ethereum does not supplant Bitcoin, nor does it try to. It complements it.
Bitcoin is your store of value, your gold vaults. When you want to put your capital to work you need an engine (Ethereum) and to run the engine you need oil (Ether).
It's completely possible to imagine (in my view, inevitable) that we'll frequently see derivatives built on Ethereum backed by Bitcoin."
https://www.quora.com/What-will-be-the-future-of-bitcoin-after-the-Ethereum-project
There is no need to worry about the future of bitcoin since the appearance of Ethereum. They have different applications.