You've said it all, there is no niche that one would want to go into that isn't already existing

What we have to do is use an already existing business of ours to earn our way up to the top

I know someone who is always living this part of the cycle.

His field is Facebook Adverting, but, he makes content videos explaining the tricks he uses on all his adverts, and gain his subscribers

So, he earns from his affiliation as an expert in Facebook Adverting, and at the same time hit the subscribers mark that makes him earn from YouTube revenue

It's a win-win