Retaining a customer is cheaper than trying to acquire newer customers. If you can retain every one of your customers, then you can keep earning revenue from them year after year. Your customer base can only increase with time, so will your revenue.
You can retain customers if you deliver what you promise. After you have met the minimum promise, try to surpass the customers expectations.
The best way to do that is to raise the service bar. With low switching costs in the Digital era, it may be the only option for survival and growth.