If the the 'rental car' fleets have enough idle capacity that people can get a car within minutes then I could see that happening. However, a more likely scenario is it will be difficult for these companies to have enough idle capacity to satisfy people all of the time.
To offer a product that is cost competitive with owned cars, what overhead do you think is necessary? While a car sharing model would have some savings from things like lower in-house maintenance costs lets assume those are zero for now and everything is overhead. Would 100% overhead (costs + profit) be enough?
I think it is very easy to demonstrate that usage is well under 50%, even at peak times. On my block there are about 60+ cars and and unless something special is happening I never have a problem finding a parking spot, and usually right in front of my door. On snow plow days (my street starts at noon) I generally need to find a spot by 11am, or I am stuck hunting although only rarely do I need to go to a different street. That tells me that in the middle of work days, more than 50% of the cars are idling at peoples houses. I would also bet that over 45% are idling at work as well. The point is however the worst peak is 50% utilization, and more likely it is less than 5% with a rush hour peak maybe 25% (400% overhead available).
Certainly my generation is attached to their shiny car, that is cultural. I am seeing much less of that in newer generations, they are more open to things like car sharing (we have 2 spots dedicated to them on my block, and they started to put in an electric spot as well but never completed it). Can we break those cultural bonds?
What is left is personal belongings we like to leave in the car. Certainly those dedicated to the car itself (eg. shovel, traction aids, etc) can be handled by the car company. That leaves things like an extra hat & glove I like to have in the winter, and jacket or hoodie for the rest of the year in case I am out and the weather changes; and of course the inevitable sun glasses. Years ago a cassette (or CD for you youngsters) collection was necessary, but that has been obsoleted by the cell phone we carry.