I'm unsure if anyone is seriously making arguments that dropping in Uber/Lyft into a city like Phoenix, AZ would be much more efficient. Rather, it would hopefully help as an intermediate transportation mode for cities that have a somewhat useful transit system. For those cities, mass transit is the highly available mode.

To state the obvious, people can want all the availability in the world, but they don't often get that because of the costs associated with it. Cars are expensive[1], and so is parking[2]. The costs of congestion are also typically only paid for by drivers with time (although many cities are toying with congestion pricing). Of course, the cost calculation for residents changes for different cities; the costs in Manhattan vs in Atlanta are very different.

So while you in particular may be able to afford the private cost burdens of car ownership, I believe the convenience of owning a car is a woefully inadequate metric to make policy decisions on. This is all mostly moot anyway as at least according to the original post, ridesharing does not positively impact car ownership.

[1]: https://www.hks.harvard.edu/publications/64-billion-massachu... [2]: The High Cost of Free Parking by Donald Shoup (ISBN: 1884829988)