When people talk about diversifying your portfolio, they do not mean to sell some of your bitcoin to buy a bunch of shitcoins. (Or, if they do mean that, then they are idiots and you shouldn't follow their advice.)
It is always wrong to look at it as black and white, it is true that all altcoins are less valuable than Bitcoin, but as an investor, in certain times some altcoins achieve a return on investment higher than Bitcoin. If you are a short-term investor, then distributing your investments to some altcoins means more liquidity will flow to focus on buying more Bitcoin, and this is what I mean by diversifying the investment portfolio.
• Diversification within a highly correlated market sector does not provide liquidity neither does it help reduce market volatility. As you mentioned cryptocurrencies are one asset class and contrary to the op, this doesn't make it ideal for all types of investments as majority of the coins are cheap replicas of others, while a lot more are scams.
As I explained, I divided these currencies into CLASSES BTC/USD(T,B,C) pair is not linear, and if you have enough cash or withdraw at certain times, you will get more liquidity.
How are ethereum and ethereum tokens special, they are all altcoins, or use individual coins in the example. Out of almost 10000 cryptocurrencies that are existing, many of them are scam while many are risky to invest on. I can not say some will not bring profit, but it is better for people to learn very well before investing on any cryptocurrency.
No, any problem with Ethereum (ETH native) means a problem with all ERC-20 tokens and vice versa.
the increase in Ethereum fees means that all other networks will increase in value, which is what happened to BSC.
You wrote so much words, yet you didn't describe how to diversify.
I divided these currencies into several classes, if you want to diversify your cryptos you must contain some of these categories. Buying Bitcoin, Ether, litcoin, Bitcoin Cash does not mean diversification because they are in the same category.