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.
Both etherium and bitcoin are actually the best and it does not really matter if bitcoin is in the first place beause what matter the most is their usefulness on why they are built. If eth can surpass bitcoin , then let it go this way . im pretty sure the supporters for both coins will still go on their own same way and not based on how the ranking was.