ETH is better than BTC in terms of it's system or usage, but what makes BTC more powerful and popular is its natural feature, there's no changes, and still the same BTC that we love even before. I consider it as the money in crypto that everyone prefers to hold and trust, and I don't see it will be replace in the future, and although ETH has a lot of usage there's also a lot of altcoins in the market now that are competing and more in the future but that would not make bitcoin obsolete because it's what people chooses to hold and trust even more.
The dominant rate might drop but it will still lead in the market.
That makes sense. After all, a Blockchain with a single purpose, seems to be much more stable and resilient for mainstream transactions. Bitcoin being the first cryptocurrency ever created is a Blockchain which fits the bill nicely. While Ethereum is capable of doing much more than Bitcoin, I believe that both cryptocurrencies will remain in the space for the foreseeable future. Bitcoin serves well as digital money, while Ethereum serves well as a platform for decentralized applications. Both have their respective advantages/disadvantages, which is why one cannot replace the other.
Still though, if Ethereum becomes battle-tested over time, it could become Bitcoin's greatest successor yet. As far as I know, Ethereum is much more flexible than Bitcoin, as it allows developers to expand its use cases in many ways. With Ethereum, there's no need to start a Blockchain from scratch as developers can create their own tokens secured by the ETH blockchain. All that is needed is some knowledge related to Solidity (programming language), and you're good to go. In contrast, you cannot do this with Bitcoin forcing you to create a Blockchain from scratch (or a fork) that would lack the necessary level of security and reliability to keep going from the start.
Despite the many benefits Ethereum brings to the mainstream world, it's best for both Bitcoin and Ethereum to head their separate ways for the foreseeable future. Time will tell us whenever smart contracts will truly keep their promise, or not. No matter what happens, it seems to me that Blockchain suits better as a financial application than anything else (as smart contracts have been proven to be extremely buggy). Just my opinion