Design Sprint are not meant to replace traditional UX agency work: they come before the actual projects even start. This is a brand new field.

In my experience of running various Design Sprints, they are great for:

• Kickstarting a project
• Pushing an idea far enough to learn from the users if it is worth investing time, skills and $$$ in building the real thing
• Get the buy-in from investors
• Bring the stakeholders and the designers at the same table and start the collaboration in the best possible way (DS are a great introduction to UX and HCD)

In my opinion, sprints are not stealing any classic UX related jobs, on the contrary, they help projects become real and ensure that they are worth it.