I suspect I'm one of the few people here who've used a "car" sharing service. I have a tiny subcompact car and some large hobbies and home ownership tasks so I rent the home depot truck on a fairly regular basis.

Something not considered often is an old telecom problem of trunk utilization based on random access and special event access using Erlangs of capacity per hour or something. My wife used to program PBXes and its quite complicated and interesting corner of math. There being a car there for you when you want to leave is both a very complicated and very calculable problem and its possible to calculate something legit and honest not just make up numbers or run simulations. With respect to my "car" sharing at home depot it gets to the point of calling ahead and reserving the truck and starting the rent meter before I even leave my house... I drive a cute little Yaris what am I going to do if I buy 2500 pounds of paving bricks and the truck is rented out from under me while I'm standing at the cash register? They're not going to load the pallets into my trunk... This is a problem for car sharing at supermarkets and the like; its 90 degrees outside and you just bought ice cream and there's no rental car within 5 miles because of some sportsball game on the other side of the city. Hmm. So the parking lot is still full of cars waiting for drivers. I would not be comfortable going somewhere without having an iron clad plan to return.

Another aspect of children is I have sad news that all your problems are not over when the car seats go away. Now you "must" be there to pick them up at a certain time or the police are called, etc. The cross country race isn't going to wait for you while you wait an extra 30 minutes to get a rental car. And being a very well paid and very busy dude, that 30 minutes as a fraction of my total time off per day or week is worth a hell of a lot of money to me aside from family friction vs scheduled activities. Suddenly "old faithful" the indestructible and infinitely reliably privately owned car is looking very attractive. How much would you pay to get an extra hour of relaxation per day? Ask a new parent for a likely shocking result. Cars are expensive, but worth every penny and more.

I find rental stressful. The clock is running! Unload those bags quicker. This railroad crossing just cost me $2.75 in waiting. I gotta return this thing in 30 minutes or pay another hour and I'm 25 minutes away. Maybe people burning irreplaceable hydrocarbons should be stressed out. But again, I'm well paid and have little time to relax and stress reduction is valuable to me an I can certainly afford it, so I could see buying my own used beater truck to haul stuff around once a month or whatever. I don't think I'll do it, but its not that unreasonable. You only live once and getting stressed out about stuff that is a solved and affordable problem isn't worth it.

A final problem which doesn't apply to me and my "car" sharing truck is if you hyper optimize car sharing until it actually works, abstract out the task of driving, etc, you end up with bus and taxi service. All that mental self pleasuring to merely poorly re implement something that worked a century ago. Skip all the product development and upheaval and just get a monthly bus pass and be done with it. You'll get faster more reliable cheaper safer cleaner service.