There are very few Richard Bransons. But there are lots of millionaires who just did one thing and kept expanding in the field and growing bigger and bigger in related fields.
Sure I know it's one example, and I have no idea how the numbers would stack up if some kind of research was done on this. All I'm saying is that this is not necessarily good advice, and from a learning perspective potentially counter-productive.
But look, let's keep this in perspective. This is a blog post by an entrepreneur with a new start-up. This is nothing more than press, publicity, visibility, mind-share, whatever, designed to get people talking, discussing, linking. So to be taken with a pinch of salt at best.
