I think this is what Uber/Lyft surge pricing does -- I've never been unable to find an Uber when I want one, but sometimes it costs more than I want to pay.

Many rides aren't so time sensitive that they can't be shifted to another time if the price is right (or wrong).

Car sharing companies will likely come up with a subscription service where you pay $X/month for your normal commute, then they can make sure they have enough base capacity for the regular rides and the on-demand rides can use demand pricing to ensure that everyone can get a car when they need it.