As in all best practices it is advisable to diversify with any type of investment. So even with bitcoins and alt.
Always but don't be afraid to lean heavily in one industry if you have in depth knowledge and experience. If you know crypto you have more of an edge rather than buying restaurant stocks.
I also find investing to bitcoin only is also good, you can diversify but it's not only altcoins that you can divide your capital. There are some industries that are outside crypto that you can also invest. There's more action in bitcoin but if someone doesn't have an idea on how risky it is on both sides (bitcoin and altcoins) then he should stay only with a single investment like bitcoin.
I don't believe that investing all your money in crypto is more dangerous than investing in anything else. THere's always a chance of failure in everything you do. You can fail by getting cancer and dying, being killed in your car while driving home, getting robbed. To me cryptocurrency offers a very high probability of success. Much greater than real estate and other so called safe investments. When you're poor you have to take risk or you will never achieve anything.
But you have to be safe on what you're doing and it's still up to you if you'll invest all of your money. May I ask if you did?