Sure they may trust you and depend on you for food and shelter, but they would know no different if they were loosed to roam wild.
Being turned loose to roam wild isn't a choice that is before us. What is before us is how to 'dispose' of the ones that have spent their life in our service.
I acknowledge that it is harder to spend dollars on a vet to have the animal put down and then to dispose of the body either with rendering truck or with a backhoe. I acknowledge that it would be 'easier' when the horse breaks down and can't work any more to just haul him off to a meat auction and leave before the ugly stuff happens.
I just find that 'easy' way out to be abhorrent and not worthy of the loyalty and life work those horses gave us.