Sherlock is right. One should only fill their heads with what matters to themselves. With our level of science, it has been proven that Sherlock was wrong. The brain's capacity is not as limited as he thought, but what it applies to is on our time. We have limited time. So limited, in fact, because we waste so much of it. This fact will never change and applies universally. By choosing how we spend our time determines the life that we will live.
It also applies to what we constantly fill our minds with. By feeding it with the content that we consume, we either nourish or poison our mind. By being conscious with what we fill our "attic" with, we can either be the best we can be or the worst, devoid of any purpose in life. What we tell our mind also matters as it determines what we see ourselves as. How we see ourselves is reflected on how we act which determines the kind of person we are.
One obvious example that I know is a bad habit yet I still occassionally do is "participating" in social media. Personally, I am in social media not because of being a social being, but for the mind-numbing content that exist that I mindlessly consume. This is a habit that I am changing as I have realized nothing is more harmful yet elusive as it. It is a drug, an addiction, that concels itself as harmless but damages my life in some way.
Sherlock is right on 6 points.