I don't see why they can't run the Minnie Vans backstage at some point in the future. Then you don't need to keep unfilled buses on schedule. You also don't need the CMs waiting back there. You can use the Minnie Driver since you know your guests should be waiting just on the other side of "backstage" for an on-demand service. The driver hops out and escorts them to the vehicle.
I expect we will see this, with an upcharge for each "backstage" pick up or delivery. It gives them more flexibility, more ability to upcharge, and less expenses in having unneeded vehicles and CMs standing around. Probably be a while since the Minnie Van rollout seems very deliberate.
But if they couldn't fill the buses, but still see a desire for this premium service, this is absolutely the way to go once the infrastructure (enough Minnie Vans) are in service.