Bitcoin and Ethereum doesn't seem to be mutually exclusive and I don't even think that at this point they are necessarily a competition to each other. Bitcoin is the leading store of value solution and Ethereum is doing an entirely different thing with building a platform for DApps, DeFi etc. I don't think Ethereum will replace Bitcoin but I also don't see any crypto replacing Ethereum at what they're doing in the near future.
Bitcoin and Ethereum can co-exist, because the purpose is different for both these cryptocurrencies. Bitcoin is more like a pure cryptocurrency, while Ethereum is a smart contract platform. Not just these two can co-exist, but there is even a chance that they can supplement each other. The developers of both these coins needs to work together to achieve that.