If your agency is towards the low end of the client vendor value chain, and by that I mean execution-focused, your services are always in danger of being unecessary. How can you avoid that?
The goal is to become indispensable in theory and flexible in execution. Specifically, you should move up the vendor value chain until you reach a node that is never not required. And then park your focus at that node. If you get all the strategy from that node, then you can control the execution downstream. But, you have to remain flexible, not favoring any particular mode of execution, in order to capture all the downstream projects, too.