Cheaper, mostly-automated long haul trucking should lead to an overall increase in its usage, meaning there will be more long haul trucks to unload than currently, which will then require even more medium sized trucks (multiple per load) to transfer to. This is a simplified version of the prevailing current theory as I understand it, and it seems fairly solid to me.

There's no denying that full Level 5 across the board would lead to very large disruption, but it's a little too early to call how it will play out yet.