Really this advice is born out of two things: 1. Many investors argue you should build your product in a narrow vertical that has the opportunity to turn into

Really this advice is born out of two things: 1. Many investors argue you should build your product in a narrow vertical that has the opportunity to turn into a much larger market. Given you start in a narrow vertical, you should aim to dominate that vertical ('Crossing The Chasm' makes a very coherent argument in favour of this). 2. Investors are aiming for 10-100x returns, and it's hard to do that if you are not going to become the clear market leader. Whether this leads to a better society is an interesting question. One might argue that it makes your company more likely to be successful, and successful startups are job/value creators which is better for society.