How can the scrum master respond to a stakeholder who suggests using velocity as a...

Velocity of a Scrum team will stop once it becomes stable and only then the team will become efficient with its deliverables.

Velocity can only measure if a Scrum team has a stable and predictable run rate but not whether or not they are delivering better deliverables efficiently.

Velocity will become more predictable only in stable teams and velocity should not be measured in Scrum teams until that occurs.

Velocity is a great way of measuring the Scrum team's efficiency, and a stable velocity means the team is predictable and efficient.