Decommission is intentionally a brief, event-style phase, and its whole point is to be anticlimactic. The reason it can be is everything that happened during Overlap: users were trained to access history through snapshots instead of the legacy system, coverage gaps were stress-tested and closed, and confidence was built — so by shutdown day, daily work already runs on snapshots. For end users the lights simply go off. Internally, it's the trigger to retire the legacy infrastructure: licenses, support contracts, hosting costs. It earns its own phase in the model even though it's brief, because external audiences need to actually see that the legacy system gets shut down — it's a milestone, not a continuous activity, and 'non-event' is the success criterion.