*Blows dust off account for my biennial post...*
I believe staff are needed on board the trains for DBGT as a safety thing as well. Guests have to remain seated during the ride, and it's much easier to control that with a staff member there since there's no restraints. It can obviously be run without staff on board the train (engineers do that during testing), but presumably that's in a sort of manual mode which Merlin would never allow with guests post-Smiler.
If they did get rid of the VR headsets, introduced some sort of restraints system and kept everyone on the train for the whole ride (ie - didn't make guests leave for the tunnel scene), I guess they could have the ride reprogrammed to run without staff on the train. But that requires a huge effort to change the ride's focus (you lose the illusion aspect for example), and there's big questions about how you keep 58 people entertained for 6 minutes in that train set up.
It's not an impossible task, but the size of it should not be understated, and one would have to wonder if it's worth the effort.
I'm by no means an engineer, but I don't see any particular reason why the building as is couldn't be repurposed for another ride. There's a pit which houses the motion base and helps with the illusion of the hanging carriage, but the rest of the building is effectively on one level. I don't see why that part of the building couldn't be used for a (more simple) tracked dark ride for example, especially if they were clever with the space.
Again though, something like that is not an easy task, and I imagine that it would be expensive to remove the the ride hardware. Then you have to take into account the actual design and construction of a new ride in its place. And making it a ride that Merlin would be willing to invest in. And consider the fact that Thorpe have invested minimally over the past few years, and there's arguably more important needs across the park.
There's no easy answer on how to solve Ghost Train.