Babaoriley: Rainer, for the vast, vast majority of investors, it's too difficult to know as much as one needs to know in order to follow what WB says.

I thinks that's true, Babaoriley, and therefore for the vast majority of investors it is absolutely okay to diversify. However, if you really know what your are doing, there's no need to diversify at all cost. Everyone who is building his own business, investing money, effort, and lifetime will likely agree on that.

The crucial question is whether you really know what you are doing or if you only think you know what you are doing! It's not hard to fall victim to the Dunning-Kruger Effect!