While most people replace old HVAC systems to save money on utilities, there are other safety reasons to get a new unit. Carbon Monoxide is a very serious issue with old systems. Old furnaces can crack and carbon monoxide can leak out. This leak will make its way into your home. The older the system the more likely this is to happen. This is a silent threat that only a carbon monoxide detector can detect.
Additionally, there are electrical issues that can be a cause for concern with old systems. When they get old, fuses can short or the connection points can weaken. If you're lucky, this will simply cause the A/C or heat to stop suddenly. The worse case scenario? Fire. Maintenance is important to look for issues and address them as they appear.