I believe that Google's long term plan is to get people to give up their cars and turn the transportation of people and things in to a service industry. From their investment in Uber, their robot vehicle projects, their buying spree for robotics companies to their crazy balloon internet service - it all points to one thing. In the future Google want us to stop owning cars, stop travelling in cars as much, and to be able to live and work anywhere and still get all the things we rely on our cars for. That includes short trips to the store, and that is what GSX replaces.
Even if Google lose a couple of billion dollars a year subsidising delivery and taxi services for the next decade, the eventual payoff will make it worthwhile.
Put simply, Google has enough money, and consequently enough time, to build a service that replaces the multi-trillion dollar individual-owner car industry.