I think that Cardano will not replace the ETH. No more coins can replace ETH. Maybe the one who will replace Ethereum will appear in 2019.
Ethereum is currently in a bad condition, the value is so low that many people say that the Blablabla coin can replace Ethereum. Even though Ethereum is currently not in good condition but he will be stable again and he will rise. Ethereum is the best Altcoin for now, no one can replace it, maybe in the future they don't know when it will happen.