It will be difficult to move the first cryptocurrency in the World! Investors are more interested in Bitcoin as the accumulation and storage of $, and Ethereum is more attractive in terms of regular transactions due to small fees and decentralized applications for developers based on the Ethereum system. Therefore, I believe that Bitcoin will be No. 1.
Good point. Bitcoin's store of value properties will surely catch the eye of many investors over time. Unlike Ethereum, Bitcoin has a limited supply with a constantly decreased block reward. In no way, could Ethereum replace Bitcoin as it's meant to be used as "fuel" for the world's decentralized applications. With an unspecified max supply, Ethereum could experience a drastic inflationary rate over time. We all know that Ethereum serves well its purpose as "digital cash" than anything else. While ETH might not be able to replace Bitcoin after all, it could very well complement it in the future. It's no secret that Bitcoin is only useful for finance while Ethereum is an indispensable tool for the launch or creation of censorship-resistant dApps. Where Bitcoin cannot fulfill certain tasks, Ethereum will fill the gaps. The advanced features of ETH greatly expands Blockchain's capabilities like never before. However, it's that complexity which makes Ethereum somewhat unreliable (unlike Bitcoin).
In the end, it's believed that Bitcoin will remain in its rightful place on the market (as the #1 cryptocurrency of the world), while Ethereum maintains its second place in market dominance. With the ever-increasing development and innovation from both sides (ETH and BTC), you could expect the Blockchain industry to improve like never before. After all, Ethereum's ambitions is to become the "world computer" instead of the dominant cryptocurrency of the market. We'll just have to wait and see what happens within the crypto space over the long term, in order to determine ETH's true place within the mainstream world. But so far, it's been doing good retaining its rightful place on the market as the second largest cryptocurrency by market cap. Hence, both BTC and ETH will grow alongside each other for many generations. Just my opinion