Sukie and I form a household of two and we have a single small car. In theory it can seat four, but that would only work if the two in front were under 5'6" and those in the back were short -- certainly less than 5'0". Effectively we have two seats.
People tend to buy a vehicle by thinking about a near worst case need. What if you have to haul six kids around --- minivan or SUV ... what if you intend to do some home repair and need to haul 4 x 8s ...., what if...? How frequently do you have these needs? The mix of family size, suburbia and work locations create legitimate needs, but I suspect many are buying over capacity.
About one percent of the time we need something larger That's where delivery, friends or car rental applies.
We just learned a friend will be visiting for a few days at the beginning of July and we'll need to do some driving with her. Since she is taller than 5'4", which is about the limit for sitting sideways in the rear seat, it is car rental time.
Do you buy a car or truck for the worst case scenario? I wonder how many people justify an extra large vehicle to cover under one percent of their transportation needs? Having fewer cars can save an enormous amount of money - given the cost of inconvenience, what is a good compromise? For us it would be nice to have something larger about two percent of the time, so we're close to 2σ (actually we're just looking at one side of the curve, so we're looking at z > 2).
If a large number of people decided to save money by getting rid of their largest car, one can imagine car rental companies doing well. Perhaps they could sell subscription plans. But a drop in the number of vehicles per household seems unlikely.