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.
maybe etherium is a better coin but don't forget that bitcoin price is tempting every people to keep their bitcoin so I think people will hold their bitcoin as long as they can. beside that maybe people will sell with small or limit amount because they know that bitcoin in future will be expensive and they waiting that time to sell. there is few people that thinking about better coin like ethereum or the other coins but they are only thinking about the price of the coin. the most expensive coin, they will save it and the other coin, they will save it too.