The wisdom of the late Arthur Ashe, Wimbledon champion 1975
Service design typically starts from a high level view of the customer journey but that's not the only starting perspective. You can also start from an individual touchpoint and connect outwards from there to understand what antecedents and descendants exist.
Start where you are
In a technology context, we typically begin our understanding from the perspective of a software project. Most software projects are Trojan horses for behavioural change, whether this is known or not. Software that is designed to be used by human beings has a ripple effect in terms of the resources consumed and outcomes created
You can't always choose the ideal starting point. Even if the conditions were ideal this might have the effect of at worst making the execution fragile in response to change or at best over promised.
Use what you have
We don't always have the ideal mix of skills and capabilities to hand when a need arises. That should not put us off pursuing the bigger picture. Whatever you do know can connect to what you need to know. Touchpoint design is an important consideration in the adoption of new technology.
Do what you can
Context you can reasonably control decreases with complexity. Likewise, the further you zoom from a tangible expression of a touchpoint, the less certainty you can have about your understanding of what will happen.
Service design that begins by anchoring itself to a known quantity is built on firmer foundations.