For example, t-shirts were invented forever ago. Plenty of people start t-shirt companies and make money doing it. Hanes sells WAY more t-shirts than you will. It doesn't matter.
An established market is a better place to play than a totally new thing. You can be a very small operator and still make a very good living for yourself.
Think about Basecamp as another example. Microsoft Project owned project management. Basecamp came into the same market with a different angle and made a business out of it.
Traditional wisdom says stay away from existing big fish. I would say swim right next to them or in their wake. There are always little subniches and different ways you can sell the same basic solution in a different way.
Another way to look at it is software is a bit of fashion. People get tired of the same thing. So, sometimes they go buy a new thing that might have one feature they care about that the old thing didn't.
Most huge products you know about, use, and love started in niches that had established leader. This is true for iPhone, iPod, Mac, MacBook, Chromebook, Github, Facebook, Google, Twitter, MySql, Firefox, Chrome, Android, BlackBerry, Galaxy S, Xbox, Playstation, GoPro, and on and on and on and on and on.
Do yourself a favor, make a pitch page, have a beta sign up form, and start finding people to sign up for your beta. It doesn't even have to be a built product yet. It can be unfinished.
The only thing you need to have a business is customers that pay you money. You figure that out and it doesn't matter what niche you are in.