Degeneration. One of the most common issues young patients that have spinal fusion is the development of "adjacent level degeneration" which is the breakdown of the segment above your spine that was fused leading to more back pain or nerve issues. This occurs at around 3%/year so in 30 years you wound have an average of 90% chance of having another problem. You should discuss with your spinal surgeon.
It depends on . Several factors: the diagnosis, age and health of the patient, the extent and type of surgery, if the patient is a smoker, if obese, presence of osteoporosis/diabetes/thyroid disease and the nutritional status and conditioning of the patient as well as ongoing conditioning of the patient. With all these variables, you really need to discuss potential outcomes with your surgeon.