That applies when the idea is something genuinely challenging.
But what if it's just a specific need felt by a specific group of people that can be fulfilled with a CRUD app. That's a viable business, but the product can be implemented by just about any developer.
Anyway, an idea that a) has the concrete people with a concrete need, b) can be (at least initially) solved with a CRUD app and c) hasn't been (successfully) tried before is so rare that you probably have more chance to literally stumble on a buried treasure. Of course we would all like to have an idea like this, but the problem is that you can't possibly know that you do until you execute and test it -- in nearly all cases it will turn out that you were wrong on b) and/or c) above.
But it really does work that you figure out a group of people with a need, and then fulfill that need the way they want. BingoCardCreator is a good example of this. There are countless opportunities if you just find them, and even having competitors in the space is not an obstacle - everyone does sales anyway.