I've worked alongside dozens of consulting firms. Exactly once, I saw a partner proactively tell a client: "You don't need us for this next phase. Your team can handle it. Here's how." She lost about $400K in extension revenue. The client has hired her firm for three subsequent engagements, each one bigger.
Takeaway
Trust is the most undervalued asset in professional services. Most firms optimize for utilization. The ones that optimize for trust build something that compounds. I try to operate this way every single time: stay until the work sustains itself, then leave. If the work is good, the phone rings again.