I’m not sure it would add anything to Duel if it were trackless. My experiences with trackless rides are based on Symbolica, Antartica, plus the Disney classics (Tower of Terror, Universe of Energy and The Great Movie Ride).
With Symbolica and Antartica they have these scenes where the cars dance around each other, but with Duel part of the appeal is that you’re ‘alone’ in the haunted house. I think seeing other cars would ruin it a bit. To make sure the cars don’t end up in each other’s sightlines they have to pass through each scene quickly, so there wouldn’t be much time for them to stop, or double back on themselves. Some trackless dark rides have simulator elements, but we’re meant to be going through a haunted house where the floors would be flat, so having the seat bouncing around isn’t really necessary.
I think the Haunted House was a masterpiece. It’s slightly baffling that they ever wanted to change it, aside from the fact they wanted a cheap new ride to market alongside the hotel and water park.