If look at this picture, it turns out that diversification is not such a good idea.
Diversification doesn't truly exist in a highly correlated market. The crypto space in one of such markets where currencies mirror the price actions of others, so when there is a crash majority of coins get effected, similar to when there's a drop.
A diversified portfolio would cnfaim assets from different niches, real estate, crypto, stocks, gold etc.
To do this, need to have the necessary skills, abilities and knowledge, which most likely will be lacking. In theory, your advice is good, but in practice, few people are willing and able to follow it. Especially when it comes to newbies. Many will prefer a simple and easy way - investing based on the capitalization of the cryptocurrency on coinmarketcap, that is, the top-10.
You should not be investig your hard earned money, if you can not take out the time to actually research on what you are investing. If one blindly jumps into an investment, there is a high possibility you would get rekt in the long run.
Researching is not rocket science either.