People need to stop believing in this TOP 10 fairy tale. if an altcoin suffers a lot of pump and dump it will surely be in this top 10 even though it is shitcoin, so this top 10 should not be something of a metric to say that an altcoin is good or no, on the matter of diversifying, if someone buys a lot of cryptocurrencies that person is not diversifying the investment, that person is just taking more risk of losing money. to diversify would be to have a cryptocurrency, to have other assets like gold and business in the real world
I agree with you that there must be variation in purchases but in that case you must think about which particular asset need to buy. I think if anyone invest in top 5 coins from top 10, the chances of losing are relatively low. In this case, there is a high possibility of increasing the price. one or two coins may be fallen in risk, but it is not possible for top five to collapse together.
top 5 should also be seen that if we look at the picture given by the OP, it is clear that in the top 5 there is at least USDT and USDC there which incidentally is a stable coin which is not very suitable if you say investment for the long term.
They are indeed good enough for trading but when it says Invest it is clearly not a good enough option.