I read an article about a serial entrepreneur who had a tendency to succeed greatly at his businesses. He said roughly the same thing. His way of putting it was that you can have it all, but not at the same time. According to him, he did like you by focusing everything on the one business before moving on. He would sell his businesses after making them successful.
I like that coming from the entrepreneur, that's where I think focusing at one thing at a time comes in. From what I have learned by experience, if I have many things to do, I will rather focus my energy on one before moving to the next.