[repsilat]

> When this happens, statistically the closest driver will take X less time to get to you.

If everyone is spaced evenly around the place and there's no congestion, you need four times as many drivers to halve response times. For a while you get good gains per additional driver, but as the numbers increase the marginal benefit tails off. This means a new player with some capital can likely compete (and is probably what the grandparent meant by "saturation".)

[amenghra]

The same thing happens from the driver's point of view. Each additional rider adds a marginal value to any given driver; there's only so many rides they can make in a given day.

In most two-sided markets, the ratio of users on both sides plays a more important role than absolute numbers.