Anything can happen in 10 years, its a very long timeframe to talk about things. Will eth replace btc? what if eth get replaced by other cryptocurrency before 10 years? someone had suggested earlier that dot could be eth replacement. so really, there is no way to know the future. It depends on many things that can happen in cryptocurrency.
Everything is normal when altcoins swap positions (XRP replaces ETH), the market isn't too volatile. However a coin including ETH replacing BTC position can lead to huge fluctuations, I don't believe this happens.