The cab can be a lot cheaper, if you don't have to keep a human comfortable and safe inside. Also keep in mind that an autonomous truck can drive 24 hours a day, and doesn't need to take regular breaks throughout the day nor sleep. (They will need maintenance, but probably not more than a manned truck.)
You are right that the costs are real. Things like first/last mile (or loading and unloading) would probably need a major reshake of the industry, if the truck doesn't bring its own labour, in the form of a driver, with it.
Amortized over the life of the truck they really aren't.
Heck, maybe they don’t even need to be articulated. They could look like slightly larger Amazon or UPS delivery vans. No need to worry about jackknifing.
If it’s a fleet maybe battery packs can be swappable so when it arrives at a hub to load/unload it can pick up a charged battery pack.
The only issue is for cargo that can’t be split up.