People often pay you to learn, but that's not the point. To answer the question you posed in the earlier post about domain knowledge: you focus smaller and smaller until you find something where you can become an expert in a reasonable time.

e.g., "agriculture" is not a niche. Breeding management software for Silver-laced Wyandottes is a niche. You reduce the huge term "agriculture" by going agriculture->poultry->poultry husbandry->poultry breeding->breeding-chickens->breeding-Silvery-laced-Wyandottes.

Now, is there a market for that product I picked out of the air? I don't know, but I suspect, based on my knowledge of the field, that there could very well be. And you could own it if you wanted to.