You can say that Bitcoin represents all the cryptocurrencies, because it was the original cryptocurrency which was created by Satoshi Nakamoto. Almost all the other cryptocurrencies have copied the basic traits from Bitcoin. At the same time, I don't agree with your argument that Ethereum represent the altcoins. Ethereum was not the first altcoin that came to circulation, and it is not the most advanced (in terms of technology). Namecoin was the first altcoin, created in 2011. Ethereum ICO occurred in 2014. And Ethereum is a second generation cryptocurrency. Now we have 4th generation cryptocurrencies circulating in the market.
I believe the logic is that ethereum has innovation that bitcoin doesn't have, which is why it is "representing" other altcoins in a way. Bitcoin was the first crypto, and there were ton of other crypto between bitcoin to ethereum as well, but it was ethereum that took the crown of altcoin king by becoming second, and did not leave that position for years now and they are still working on improving it more and more, in a week or two we are going to see a huge dump in the gas fee prices as well which will be great.
So long story short we are talking about something that is ahead of all other altcoins in price, but also giving them examples as well. Before ETH did it, no other coin (as far as I can remember) got used so much for its blockchain, there are thousands of tokens created based on ethereum blockchain, just look at how much erc20 is used and you will see it. Others do have it too, but ETH paved that road for them.
