Now, if the needs of your online 'neighborhood' have already been met, then that's similar to living in a neighborhood full of Italian immigrants and still looking to start an Italian restaurant nearby. You'll have to move and learn about the needs of other 'neighborhoods'.
If you have a business that depends on Metcalfes law (the value of a network is proportional to the square of the number of connected users of the system) then yes you need to get big fast and become the market leader.
But for many other types of business this is simply not true. To take 37signals as an example there are plenty of players in their market, probably thousands that are turning a tidy profit. I can name a few just off the top of my head.
That same neighborhood sometimes wants Chinese, or Greek etc, etc.
Given those two statements, and the sheer numbers on the internet, it will be a long time indeed before we reach the limits you talk about.
That being said, if you open an restaurant and serve canned spaghetti, you're probably not going to see high levels of success.
This metaphor is bad. On the internet, there can't really be that many Italian restaurants because there are no local monopolies and no marginal costs.* On the internet, there would only be one Italian restaurant, and it would be 37 signals.
*Sure, there's the speed of light and the cost of electricity, but these are not strongly limiting factors.