In a 100% ideal scenario an organization would only need developers as every possible failure/problem would be covered and handled by automation.

Obviously the real world is different. New code is deployed that has bugs, there are unpredicted events, there are complex failure scenarios that are difficult to automate etc.

A DevOps engineer may write automation software in conjunction with developers to automate the operational aspects of business logic. Thus, it's a partnership between the DevOps engineer whose metrics are driven by availability/reliability/scalability/security and the developer who is trying to attain some business objective.