Must you focus on just 1 niche?

A reader wrote this to me:

“My passion is to help people lead a happy life. I want to help people achieve fulfillment in 3 important areas of their lives… relationships, finances, and career. 

As I’ve read and researched, I see that many experts recommend tailoring down to a specific niche, and focusing on narrow issues; thereby warning against taking a big range of topics. I perfectly understand that the 3 areas I’ve picked are so wide! In fact, each has countless sub-topics.

But what I’...