Etherium is a better coin since it also has the smart contract feature. There is nothing that you can do with bitcoin but you cant with etherium. So why not all people sell their bitcoin and buy etherium.
Ethereum is a different thing. It offers a lot that bitcoin doesn't offer, but it won't replace bitcoin, it will expand in a different direction. Bitcoin is effectively digital gold, a store of value, and the gateway between alt coins and fiat. Bitcoin is not going away!