I think its a case by case scenario... replacing only one arch might be suitable if the opposing arch is set ideally and isn't too worn. Sometimes finances doesn't allow a patient to get both new upper and lower, so we just have to make the most of the conditions that we have to work with. In a perfect world I think by replacing both upper and lower we can get better or improved balance and function and improve on any potential problems that the existing dentures may cause. Sadly insurance companies don't see much logic when it comes to doing what is best for the patient... sometimes just gotta make the best of the cards you have to play with.