One important factor to consider is that this won't happen overnight. Once autonomous trucks actually hit the roads, there will be a long period where the drivers will still have to be behind the wheel. And even then, there is a lot of other work a truck driver does other than drive. My father runs a small trucking business, and according to him, the logistics are a way bigger annoyance than the actual driving. You have to time deliveries, you have to unload the cargo without damaging anything, you have to repair the truck (or call a tow / mobile repair) you have to take photos of any potentially damaged products, you have to make sure there are people there to sign off on a successful delivery, you need to find the right people when nobody knows anything about your delivery, etc... The job sounds pretty cut and dry, but in reality, every delivery has some new issue and the driver is usually the one that ends up dealing with it.

I'm not saying that the job won't be automated eventually, but the actual driving is only a part of the job. And even when the automation begins to happen, I think there will be time for them to transition into new roles.