The vehicle subscription would replace the passenger car, not the truck used for heavy work, not at the least because the depreciation of the work vehicle is deductible as a business expense. Also, a vehicle actively used for construction work is already heavily utilized (it is hauling/holding stuff most of the day), so it makes more sense to own it. Passenger cars, however, sit idle most of the day in a parking lot or your driveway, hence their low utilization / wasted value.
Also, many cars today have built-in car-seats (https://www.cars.com/articles/2014/05/which-vehicles-offer-i...). You can bet this sort of feature will be more common in autonomous vehicles.
Furthermore, in certain Uber markets today, you can pay an extra fee to get a vehicle with car seats pre-installed. Right now that fee is high ($10/ride) because they're the only ones doing it, but it's a service so easily replicated that the price will surely come down.