"The ridesharing space does not offer advantages to economies of scale" - efficiencies of scale in ridesharing are massive; more consumers means a more liquid marketplace and less downtime for drivers (idle time or time-in-transit to a passenger)and more drivers means shorter wait times and increased reliability in car availability. Beyond the improved consumer experience, there's real $ value creation resulting from these efficiencies that can lead to increased revenue/unit of time for drivers, decreased fares for passengers or increased commission for the ridesharing platform.

Those are very valid points, and they certainly apply at the moment. However, I think that this ceases to be an issue when meta-rideshare services begin to emerge, which I think is (technologically) inevitable.

What I think we will see is that a driver will use a meta-rideshare to be available on all ridesharing services (Uber, Lyft, Sidecar, etc.) just in order to maximize his/her rides per hour.

The same for users, who will use a meta-rideshare to query all ridesharing services in order to get a ride as quickly/cheaply as possible.

A great analogy for this is in airlines: yes, some airlines are bigger than others and consequently have certain advantages. But most customers use flight search engines (Google Flights, Kayak, etc.) in order to find the cheapest/best flights.

The only counter-argument to this would be that users would have some sort of brand preference for choosing Uber every time rather than using a meta-rideshare to find the chapest/fastest ride. But such brand loyalty is unlikely too emerge: (1) Uber's public image is far too tainted, and (2) it's very hard to differentiate Uber's core product (seat in a car) from that of the competitors.