I agree they could do it if they wanted to.... the fact that they haven't decided to do it tell me that that the benefits of doing it don't out weight the costs.The above are all examples of Government transportation systems. They are just as big of a target for liability claims as a Disney is. The model is out there and used all the time. It's not a hypothetical - it's common practice.
The gap issue is addressed by the train's door/floor.. not precise alignment. Sure it adds precision to where they must be to load... but that is another thing that can be addressed with aides when you have a fixed environment.
I think the biggest problem for Disney is cost and desire for flexibility. I mean, they could build the entire bus depot elevated if they wanted to.. but clearly they haven't.
As for government transportation systems being as prone to litigation, that isn't always the case as governments often write laws to limit the ability of them being sued. Frankly the thing that has always boggled my mind is the typical public subway never has anything to safeguard passengers from falling off the platform to the tracks below, when it would be very easy to install the same type of gate system Disney uses for monorails... yet the government run subways don't seem to care or worry about the occasional person that jumps, falls or is pushed onto the tracks.