This is an assumption, and I would say, an invalid one. The reality is actually that there are many successful businesses in any given industry. This is thanks to the fact that one-size does not fit all. Niching down is a good way to build a small, profitable business, because you can address the needs of the niche better than those bigger players that have to try to solve the problems of many different customers using the same systems.
Its size can be as much a hindrance as a help. Big companies turn more slowly and are less likely to put out elegant ideas for the same reason a committee is.
Paul Graham writes about how his small online store was able to turn out features faster because he was leaner. I can't find it among his essays (http://paulgraham.com/articles.html) but Getting Real, a small book by the makers of Basecamp, says the same thing (https://basecamp.com/books/getting-real).
Big companies like Google know this, and often seem to structure themselves into small competitive groups.
Google is not now, nor will they ever be having a meeting where they decide to allocate a few hundred engineers and marketers to crush that space.
There are tens of thousands of niches like this that will pay for teams of 1-3 people to live on the beach after a few years of work.
But in this winner-takes-all reality, is that possible? If your business concept becomes successful, what prevents a bigger player to copy it, improve upon it, and steal away your customers?