I was worried about that at first too. What would be left over. Today I am not at all worried. There are so many interesting niches selected that I think many people are motivated by their passions versus the big money makers.
I also think that there is still a tremendous amount of niches still available, and there are plenty of niche topics that have not even been invented yet. What i mean is that every day our world invents new things, activities, trends and hobbies that develop overnight and have viral adoption.
I think a bigger question is can people turn some of these niches into viable content products with regular contributors and readers. Time will tell.