1. Purpose of the gate
Steps that once earned their place stop doing so. The work moves on and the step's accuracy drifts beyond recovery, a supplier withdraws the tool or changes its terms unacceptably, the cost outgrows the benefit, or a better option is qualified. The Retirement or replacement decision gate opens the Retirement or Replacement stage. It exists so that ending a step is a decision taken on evidence by its owner, with users protected, rather than something that happens when a licence lapses or a hosted service disappears.
The decision taken here, Retire, replace or restrict, has three honest answers. Retire and return to the conventional step. Replace with an alternative that has passed its own qualification. Restrict to the scope where the step still meets its criteria. Restriction is a legitimate outcome, and often the right one, but it must be chosen deliberately rather than as a way to avoid the harder conversation.
One consequence must be stated plainly at the gate: a replacement starts its own lifecycle at the use case or evaluation stage. Nothing carries over from the old qualification, however similar the new tool looks. A replacement that has not passed its own qualification is not ready to replace anything.
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