JB Hunt, as an example, is who is contracted to move goods. They have close to 50,000 trailers. It appears that they own around 12,000 trucks. Even if they own zero trucks now and are entirely dependent on independent owner/operator contractors and those contractors' trucks, they could start purchasing their own self-driving trucks and no longer contract-out for drivers and their trucks.
Large companies like doing business with other large companies. If JB Hunt, Swift, Allied, or any of a slew of large trucking companies stop using human drivers then the big players like Walmart, Costco, Target, all of the department stores, will simply continue to use JB Hunt et al. because they're not going to contact thousands upon thousands of independent owner-operators. They care about selling merchandise. If they're not already running their own fleet directly then they'll continue to sub that out to whoever can.