Coming from the perspective of a black-ganger, I honestly can't see a completely unmanned ship on anything but an extremely short run.
Filters will need to be changed and cleaned, as will centrifuges, turbo-chargers will still need to be water washed and nut blasted, injectors will have to be pulled.
Then there is the issues of refer-boxes, on how many trips has a compressor had to be changed? Or electronic issues be fixed? Will nay company be willing to lose possible million dollars worth of cargo because of a blown fuse on a refer box?
Then there are leaks, which are inevitable, who will fix them? Or will the bilges just fill full of water and oil? On that subject who will maintain the OWS?
Then there is the issue of cleaning the Main Space.
Granted some of this could be done by shore-side, but is it really cost effective to hire 100-200 workers to fly through an ER in 36 hours? (or even advisable)
Any competent black-ganger should know that small problems turn into large problems if they're not addressed in a timely manner.
Then on deck, decks will not stop rusting because there's no crew, even a fairly lazy deck gang can get a lot of chipping and painting done on a 30 day trip. Or again would it be cost effective to hire 50 painters to chip and paint a ship in a 36 hour port stay?
So I would say at least a black-gang would need to be aboard, so any saving from crew cost is gone, staterooms, stores, potable water, A/C and crew lighting will all be necessary.
Once you've added a 6-8 person black-gang, adding a steward's department becomes cost effective (someones going to have to cook, wash dishes, clean the galley and common areas, and I doubt any King's point or Mass-Maritine grad is going to do those chores)
Then who's going to in charge? The C/E? Not entirely far-fetched, but is our industry really ready to say the Ship's Master is extraneous to a ship's operation?
And we raise Master's from mates, (I doubt any 22 year old graduating from an academy would have the experience or maturity necessary to be a ship's Master)
Once the ship has a Black-Gang, Steward's department an a Master and mates, adding the 6-8 AB's is a minor expense.