I had an interesting discussion with one of my colleagues about projecting market size. We talked about taking market share from other vendors and substitution and all those other things you get in textbooks and expensive seminars.
I'ld rather blow the doors off and create a whole new set of users. Now this does relate to textbook economics. There is this theory of pricing bands. If you fall outside of them, no one buys. But there is the other side to that. If you create a whole new value proposition, orders of magnitude from the existing one, you will attract a lot more of a different type of user. But this is not gradual. At some point, it just happens. Think of Skype when you could call anyone's telephone for 2 cents a minute. Especially international. All of a sudden another million user piled on. And I bet the big telcos didn't lose any business at the beginning.
But how to get there? There are no easy answers. But many times you hear "But that's the way everyone does it." Challenge all assumptions. Take your team off somewhere and leave the email and cell phones behind. Start with a clean piece of paper and figure out how to build the best product for your market. Then compare it with the existing product. You will be surprised.