The problem with unreliable position information can, I believe, be resolved by placing a network of Mode S receivers on the ground... the signal received from aircraft can be cross-checked (bit like DF auto-triangulation?), and in the event of error the aircraft's navigation equipment can be updated... i.e. no cost GPS?
Think back to the days when radar was obliterated everytime a rain shower loomed over the horizon, and SSR was subject to back and side lobes.... this is just new capability. The technology is being developed and will no doubt improve before/ if it to become a stand-alone system in widespread use.