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.
If you following the cryptocurrency market, you will see that the market cap of Cardano is 1 billion dollar, and the market cap of ETH is near 15 billion dollars. So that I think in the following five year, the ETH is not replaced by ADA, the only crypto I believe will replace ETH is XRP due to the high potential. At now, the market cap of XRP is greater than ETH.