Ethereum was first released on 30th July 2015. Since then a lot of analysts have predicted the death of Bitcoin and Ethereum replacing it as the most popular cryptocurrency. And Ethereum came very close in mid-2017. Back then the difference in market cap between the two currencies was just a few billion USD. The market share of BTC was only a couple of percentage points greater than that of ETH.
But once the ICO market crashed, ETH was also dragged along with it. And from first quarter of 2018 to this day, the ETH prices have been on a continuous downward spiral. A part of the blame should be put on the ETH developers as well. They were slow to roll out the bug fixes and updates. Even the release of Ethereum 2.0 have been postponed so many times.
That's certainly true, mate. The ICO boom of 2017, greatly benefited Ethereum in many ways. The world's second largest cryptocurrency by market cap was very close to overcoming the pioneer cryptocurrency by a slight margin. By that time, many people believed that "The Flippening" would come true as Ethereum became closer to overcoming Bitcoin's market cap. But after governments decided to crackdown on ICOs, Ethereum hasn't been the same. Because of this, prices have declined massively along with market cap. As of now, the difference between Bitcoin and Ethereum's market cap is very noticeable. There's a large gap between both cryptocurrencies that's very unlikely to change in the future.
Unless, Ethereum scales for the world with PoS and Sharding, there won't be much people using it at a large scale. Without huge mainstream adoption, demand will decline as well. Ethereum has been delaying its scalability upgrades day after day, while there are now several alternatives (like EOS, TRON, and even Cardano) which are much faster and cheaper for smart contracts. The competition is fierce, which means that Ethereum needs to give "more than meets the eye" in order to topple Bitcoin anytime soon. But that scenario is highly unlikely to happen in the future as Bitcoin remains the one cryptocurrency that's secure and reliable unlike no other known cryptocurrency today.
Nonetheless, it looks like Ethereum's outcome in the future is to complement Bitcoin rather than replacing it. After all, many people still trust Bitcoin on top of other altcoins on the market. Ethereum will have its specific use cases, as well as, Bitcoin. The pioneer cryptocurrency's store of value properties will allow it to remain highly valuable for the foreseeable future. Ethereum's inflationary model (with an unspecified max supply) would devalue its "Ether" cryptocurrency over time rendering it uncapable of taking Bitcoin's throne anytime soon. Just my opinion