Even if that is true, it really depends on the risk tolerance of each investor. If you can be responsible for the cause and effect of investing in bitcoin, then there is nothing wrong with investing only in bitcoin. Diversification of assets (bitcoins & altcoins) sounds good, but if you are experienced in this investment then maybe you will be different.

So far I haven't been as successful as other experienced investors at investing, that's because I still need to develop a good strategy and plan on investing. I'm not going to make a big deal out of diversification, but I just believe that bitcoin has a better future than thousands of other altcoins.