Diversication of investment is always a welcome idea and most times knowing the right coins to add to your portfolio is the major thing here
Diversification is very good, but i do not recommend diversifying into different altcoins, they are influenced by BTC's price, so it is better to diversify into other things that are different from BTC, like real estate, gold, etc. I know that a lot of people buy BTC and a lot of other altcoins, however i do not consider that to be diversification, when the assets are somehow linked to one another.
You're right that other altcoins are influenced by BTC but let's not forget that profiting from BTC means that you need to HODL for the long-term which is quite the opposite for a few altcoins.
Most of the altcoins has the potential to give a very high interest and profits during the short term, so it depends on your investment goals, if you're a bitcoin investor and then you wish to also pursue short-term profits, then diversifying your investment to other altcoins would be a great idea...
There are altcoins that has the potential to give you a 1000X profit in matter of months and then maybe DIPs again, but the point is that you've also made some crazy ass profits from altcoins while still HODLing BTC.
I think what you should rather say is that, it wouldn't be advisable to diversify your investment to alcoins using the same investment techniques as though it was Bitcoin ( HODLing for the long-term) it is then you'll be making a terrible mistake.