I am hearing this ever since 2014, when they conducted the Ethereum ICO. Back then, I decided against participating in the ICO, because I believed that ETH had no chance of offering any competition to BTC. Earlier (in 2013) I had invested in coins such as LTC and NMC. These coins gave me good returns, but in 2014 they went down and the future prospects were not looking great. Anyway, it was a bad decision to not invest in ETH back then. But that said, I expect the market share of Ethereum to go further down in the coming years. There are better platforms out there if someone want to go for smart contracts. And the smart contract sector has not really taken off, as much as some of us would have liked.
Exactly. There are far better options out there for smart contracts. The delays in Ethereum's transition from PoW to PoS are doing more harm than good for the longevity of the world's second-largest blockchain by market cap. The more it takes to scale, the lesser it'll be adopted by people in the mainstream world. We've already seen how alternative smart contract platforms like EOS and TRON have seen an exponential boost in transaction activity and mainstream adoption. The number of decentralized applications are growing each and every day, relative to Ethereum. If the world's second-largest cryptocurrency by market cap wants to prove itself superior to its competitors and even Bitcoin itself, it would need to adopt new scaling solutions in the least time possible. Even so, Ethereum's supply is unlimited which is unsuitable as a store of value. It'll never be able to obtain higher prices in the future because of this.
While it's becoming harder to believe that Ethereum will replace Bitcoin someday, we can say that ETH will remain as the second most popular cryptocurrency in the world. The mere fact that it's constantly being developed like Bitcoin, tells us it'll be this way. Not to mention, Ethereum has a trusted branding among the smart contracts space unlike its competitors. For true reliability and censorship-resistance, Ethereum will continue to be the choice of mainstream developers. It's like the "Bitcoin" of the smart contracts world. Which means, that ETH cannot be replaced by another cryptocurrency whereas Bitcoin cannot be replaced by Ethereum itself.
Nonetheless, there are better important things to focus than speculating which coin will replace the other. Ethereum has been doing fine as the world's second-largest cryptocurrency by market cap. Instead of replacing Bitcoin, it could complement it in every way. For finance, Bitcoin is the leader. But Ethereum takes the lead for decentralized applications that extends the Blockchain's functionalities far beyond that of finance. In the end, both cryptocurrencies (BTC & ETH) will live alongside each other for many generations. Just my thoughts