In general, focusing on a niche is a good idea. If you are selling your domains, it will allow you to mantain a list of potential buyers. If you are developing your domains, you will know how things work in this area, what visitors are looking for, you will have content to publish, and a network of sites to interlink.
Yet like the monocultures of ancient egypt and those of the 18th century carribbean, you run the risk of losing all of your income if there is a depreciation in value of your niche or if a competitor with a tremendous budget decides to take to your niche.
Yet like the monocultures of ancient egypt and those of the 18th century carribbean, you run the risk of losing all of your income if there is a depreciation in value of your niche or if a competitor with a tremendous budget decides to take to your niche.