I think that Bitcoins and Ethereum are two different coins. They have their own characteristics and features that makes them convenient to use.
Bitcoin is more on being a currency while Eth is more suitable in mining, trading and other transaction run by smart contracts.
Well I think they could not replace each other because they have their own unique use.
Maybe in altcoins, some may replace other alts but between ats and bitcoin itself, I think it is impossible.