In my opinion, ETH will be number2 coin,not second BTC, bitcoin is bitcoin and no one can replace bitcoiN as number 1 in cryptocurrency, although yes eth is also a good coin but it cant take the place of btc on top.
Sure bitcoin will always remain the first cryptocurrency, but ethereum to me will remain the best cryptocurrency. So many use cases, the smart contract ethereum offers alone has place it on a pedestal that bitcoin can never attain.