No niche is 100% monetizable, as there are always variables, but some come very close due to people's natural inclination to spend money, financial niches, such as investments, cryptocurrencies, or debt recovery, have a very high conversion rate because those who access them are already looking for solutions to their financial problems, this is reflected in the cost per click, which can exceed fifty dollars on google ads, since advertisers know that a single new customer can generate hundreds or thousands of dollars in commissions.
Another niche that's a money making machine is health and wellness, but only if you focus on recurring or high demand products, supplements, weight loss, diabetes management, and men's health have enormous appeal, and people don't hesitate to spend when they believe something can improve their quality of life, the key is not to exaggerate with promises and to offer products that solve problems that don't disappear over time, for example, chronic back pain is a problem that people will want to solve at all costs.