Cost savings are one of the drivers for reuse, but not the only one. Maximum practical flight rate is another. F9's peak flight rate over the past couple years would not have been attainable without first stage reuse, simply because of the manufacturing bottlenecks on stages. And fairing reuse was openly driven by a limited capacity to manufacture fairings fast enough - it's not even clear that SpaceX is overall saving money there, but they are saving time, and their business model is based around putting as much mass in orbit as they can as fast as they can.