In my experience, whatever boundaries are set within an agile engineering group can be overridden by the priorities, roadmaps, etc. that are imposed from above by the corporate executives. Responsibilities can change overnight as staff are reorganized or laid off by powers beyond their control.

Yes. This is exactly it.

* We track velocity as the key capacity planning metric. We don't use it for performance or anything else (e.g. we don't care if it creeps or if two different teams have two different velocities).

* We then break down all of our work and estimate it. This gives us a scope.

* From there, we have a conversation with our PM. Here's the timelines you want to hit. Given the data, we need X. Do you want to cut scope or extend timelines?

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Lastly, we focus a lot on making decisions that maximize velocity over a medium-term (typically 4 to 12 weeks). There is little value in pushing hard in one sprint if it's going to have an opposite reaction in the next sprint.