Some will take on more and more of the much more complex logistics jobs like delicate loads, large loads, etc.
Some will switch professions.
Some will go to school to switch professions.
Some new professions will open up exactly because it will become cheaper to move stuff by truck. For example, if a business can now afford to move X to their location, now they can do more business. Perhaps that business hires former truck drivers.
3 million truckers in the US alone. Retire early? To live on what, the wealth accumulated in not even a full lifetime of trucking? Go to school? Who pays for it, who supports the person/family while they go to school, and what are the odds that the skill set increases so drastically that AI/automation won't just catch up with them in a couple of years? "Just" switch professions? If there was a better one they would have done it already, so they can only switch for the worse.
Given the massive number of new drivers and that half+ of them are voluntarily switching to new jobs your whole thesis here is total bunk that they can only switch for the worse.
Trucking is one of those industries where even worse than most others recruiters lie, cheat, and steal their way into getting people into the industry and as soon as they're in they mostly find there really are greener pastures elsewhere.
Are they more complex task jobs? I rejected OP's theory because no evidence was put forward that drivers will just move to "higher complexity tasks". Are they? Are we creating a constant stream of higher complexity task jobs that truckers can routinely reach with moderate upskilling and which aren't at risk of being automated in a couple of years?
We have millions of jobs which haven't changed in years skills wise and are somewhere on the bottom rungs of job quality. Yes trucking is bad, but even "just" 1.5 million people doing this for years means they have no realistically better career option. There are 50% more truckers now than 20 years ago. So we haven't really created all that many "higher complexity task jobs" so far, we just created more of the same.
When the bottom falls out on this one, it falls all at once. The moment automation comes for one trucking job, it comes for all. There will be no bargaining power, there will be just "accept even worse conditions or take a hike". Automation doesn't fix their problem, it fixes yours: stuff's cheaper. Is your theory also that all those people will just magically get better jobs like stated here:
https://news.ycombinator.com/item?id=44114878
https://news.ycombinator.com/item?id=44115072
Is the theory bunk because you jumped straight in the middle of the thread? Or because it doesn't give you the good feelies? I'm not saying it's all doom and gloom. But let's not pretend it's all roses either. We'll get cheaper stuff, and maybe the next generation of people will have better than just trucking jobs, and all it will cost is that most of the current generation of truckers will get screwed worse.