Getting readers to pay is not easy. It requires consistency as well. Readers won't want to pay for an author who doesn't have consistent uploads. I will not pretend like it's easy. That's why I included the part about working hard.
Anyone can is not the same as saying anyone will. I think that majority of writers have the capability but won't. There is also a problem of writing style. For example, my kind of storytelling is one that is popular, and I enjoy writing it. It would be different if you didn't enjoy writing what's popular. Doing so would make the task even harder.
Yeah, I will not say it's impossible for you to make money with something niche. However, it will be like climbing a mountain and a long, grueling process. If you have the capabilities, you can try something popular to get your name out there. Maybe make 2-3 popular books, and then write what you love.
I have learned that setting a routine actually gets me to spend time working on my story, even if I'm not writing words.