I don't believe if cardano can replace ethereum. In my opinion, ethereum is difficult to destroy because technically, ethereum is almost perfect. Cardano is too far when compared with ethereum technology. Ethereum is now often said to be the king of ICO, of course this makes the ethereum even stronger. Ethereum's strength is getting bigger when ethereum increasingly benefits by providing blockchain rides for projects that want to build their blockchain-based projects at low prices and easy processes. I am sure that in the future ethereum will be more potential and not easy to destroy especially by cardano. That is a bit of my idea.
With the next update of ethereum which put a lot of result to the scalability to provide the best manufacture for the business and i guess sharding of cardano means nothing. Serenity is on the way and another hardfork will come in the future.