Services have different relationships with each others in terms of dependencies, and in terms of what you think those dependencies are.
If your idea of how things work is that services A, B, and C can optionally use service D, else use some fallback process, then if D has never failed, then you've never used that fallback process. And services X, Y, and Z which rely on services A, B, and C haven't had to deal with those services using their fallback processes either. So, instead of waiting for D to fail, you can take it down at a convenient time.
This applies to services as a whole, or services within a locality, or all services in some availability zone.
Comments
Services have different relationships with each others in terms of dependencies, and in terms of what you think those dependencies are.
If your idea of how things work is that services A, B, and C can optionally use service D, else use some fallback process, then if D has never failed, then you've never used that fallback process. And services X, Y, and Z which rely on services A, B, and C haven't had to deal with those services using their fallback processes either. So, instead of waiting for D to fail, you can take it down at a convenient time.
This applies to services as a whole, or services within a locality, or all services in some availability zone.