A migration is not enforced until the legacy path is removed. While a fail-open fallback exists, the new authority is advisory — and a process that can still boot without it has not adopted it.
Cutovers love a fallback. Keep the old path "just in case," route to it on error, and the migration feels safe. But a fallback to the ungoverned path means the governed path is optional, and optional governance is no governance. The system that can still reach the old behavior has not migrated; it has added a preference.
Enforcement is the deletion. When the legacy module is removed and startup fails closed unless the enforced component is present and at the required version, the new authority is not preferred — it is the only thing that exists. Capability is removed, not restricted. Until the fallback is gone, the cutover is a suggestion the system can decline under load.