Unless the automation creates free trucks for all to use, you scenario can't really come to pass.
This is indeed pretty much the basis for these questions to arise. Not free of course, but such marginal cost that ownership would not mean much.
When instead of 50 cars / trucks, you only need 15 and their cost is shared among the local community, with the price of buying one / maintainance very low due to automation, it will make sense to have this kind of system.
Doesn't mean ownership would disappear, simply that for most people it would be more convenient / cheaper to use this system, and as such most people would choose to do so.
wouldn't this situation make people own _more_ trucks, rather than less?
Today, we already have the situation with yatchs - people tend not to own a yatch, but share with several families/people. And this is because yatchs are damn expensive to own. If yatchs are as cheap as a car, it'd be trivial to just buy one, even if you aren't using it very often.