My Answer: Little to none. If they were to switch overnight, then there would be a huge impact. The reality is, it will take years (probably decades) for all over-the-road trucking to switch to autonomous drive and even longer for in-city/regional trucking to do so. With so many specialties in trucking (construction vs goods vs flatbed vs logging), it’s hard to imagine all trucking switching to autonomous rigs all at once. It will be a gradual process.
In the end, there will be plenty of time for most of those who might consider trucking as a career to realize that it’s not a long-term career choice and find another option. There will probably always be a call for some professional big rig drivers to fill a role in some situations. Such as when systems have an malfunction and the truck must be moved manually for repairs. Or in situations where automating is not worth the financial costs (e.g. extreme specialties).