You raise some interesting and very good questions. There may not be a single answer that fits every patient. Spinal fusion for any reason is not considered lightly, especially for the older adult.
The surgeon is the best one to advise you on this question. Patient age and general health are two important factors. The patient's symptoms (especially severity of symptoms) and function are also considered. Bone density and the presence of osteoporosis in older adults are also evaluated.
Many older adults are coming through spinal fusion surgery with good results. Keeping mobile and active is a key factor in living longer with better quality of life.
The decision must be made carefully weighing all the factors. The patient should rely on family and the physician to offer guidance. Ultimately, the decision should be left up to the patient.