I know several people who charge for an initial development fee and then offer recurring monthly packages which handle new development, updates, and maintenance on a retainer. They've been doing this for years. The best part about this scenario is that since the payment is already baked in, there's really no incentive for the "agency" to clamor for more unnecessary work. They are free to be advocates for what's best for the client since they will usually end up making more money by using the extra time to bring on yet another client than they would by servicing this existing client with more custom work.