I mean yeah, its the top 2 cryptocurrencies so its probably the king of altcoins, Most of the altcoins probably don't come near to Bitcoin and Ethereum when it comes to market price so these two giants are just the best as a cryptocurrency and even on the investment, most of the investors would easily suggest Ethereum as an investment after bitcoin no problem. Ethereum popularity is just different compared to other cryptocurrencies I don't think there are cryptocurrencies that could come close to these two, especially on marketprice.
When bitcoin becomes a thing there are a lot of projects trying to replicate bitcoin's success, but Ethereum has a different structure, different supply, etc. and that's structure is probably genius having its own network, these days other projects are now replicating Ethereum creating new networks, etc. I guess its a testimony to Ethereum's success in making a genius structure that could possibly be a big thing these years.
Just to make it a bit more clear, ETH is the second one, and if we ignore USDT which is not a crypto currency but a stablecoin, then we could say that BNB is the third one and they have less than 25% of their marketcap, imagine the fact that your closes rival is not even four times smaller than you, it's even smaller. On the other hand the next biggest one after that is XRP which has 19 billion marketcap, which means it's even less than 10% ethereums market cap as well.
This type of big differences should make it clear for everyone that ETH is solid at the position it's in and there is no cryptocurrency that could take its place, not anytime soon at least and for many years.