But that would be an argument for people movers like e.g. the Ford s-Max and others, but instead we see SUVs which don't really add more space (I'm always amaze

But that would be an argument for people movers like e.g. the Ford s-Max and others, but instead we see SUVs which don't really add more space (I'm always amazed if I get into an SUV and find the internal size smaller than a station wagon). Moreover it doesnt explain the whole category of minisize SUVs, like the VW t-roc or the Volvo XC 40. Finally if you have 4 kids close to each other, realistically the oldest will be at least 4.5 by the time the youngest is born, so there is not much time they all need a car seat (maybe 2-3 years, after that booster).