In any investment, we should never spend our capital to buy only one asset, it is too risky to do. As many experienced investors say, never put all your eggs in one basket. This means that investment diversification is very important to reduce our risk when investing. As you have explained, suppose we invest in several coins, if we lose from one coin, we can make profit from other coins. Although ETH is indeed a safe project for investment,
but it would be much better if we also invest in some other coins. What is certain is not to list too many of our portfolios, because too many will be difficult to monitor and ineffective in generating profits. I think 3-5 coins are enough for investment diversification, make sure the portfolio list contains potential coins. My advice is to avoid investing in new projects or shitcoins, that's a very high risk and will likely only make us lose money.
I would go against this, I would guess that we shouldn't be really assuming that we could be investing just on bitcoin. I mean think about it this way, there is a chance that bitcoin could be your only ever investment and still would make you a lot of money. That's why I believe that we shouldn't be worried about it, bitcoin is the king. I get that gold, stocks, forex, oil, real estate, not anything could be just "one" investment.
But at the end of the day, we can have bitcoin alone and that would be a great deal, it could make you earn a good amount of money and that could make you a rich person. I am not all in bitcoin, but I could be and I would be fine with it.