Not many coins are you going to buy? I'm not sure that while you keep some of these coins they will not disappear but still if you take 10 coins, then you need to buy them not in equal parts. For example, Bitcoin 50%, Ether 10% and the rest are divided into equal parts.
the basic principles of diversification and your advice is correct, but here a person makes decisions himself, depending on what risks he is ready to take. and the amount of money that a person has is also very important. if he has only 1000-2000 dollars, then I would also buy coins for equal parts.