When you decide to take risk, then just give it all. If you take risk investing in bitcoin, then most probably you can also do that to altcoins but investing with caution is still highly advisable.
Well, it's really up to the person if he/she will invest in Bitcoin alone and not investing in altcoins. It's also not wrong if they advice people not to invest in altcoins maybe because they didn't believe in altcoins that much. It's all about the person they give that advice if they will invest in altcoins or not at all. Let's say that I am adviced not to invest in altcoins and yet I still invest in it which means that the final decision is up to me whether I invest or not.