Not convinced this will slash car sales
Overall car miles driven will increase if there's a large move to summon-on-demand, so other things being equal the practical lifespan of cars will decrease. Also people are going to be a lot less forgiving of minor faults that they'd just put with in their own geriatric vehicle, so the fleet of shared cars will have to be quite young, much as hire cars are now. And if shared-use-on-demand really takes off then demand for those ex-fleet vehicles will be low and they're going to effectively become worthless at a much younger age than current vehicles.
Hopefully autonomous cars will crash less often, so that will work in the opposite direction and reduce new vehicle sales.
All in all I don't think it's at all clear that this new ownership model will be bad for car manufacturers. It might well turn out to be another acceleration of the disposable consumer society that's been developed for other consumer durables.