The difference between success and failure of large projects comes down to technical leadership. I've seen it time and time again. Projects that are managed by external consulting companies (name brand or otherwise) have a very poor track record of delivering. An in-house technical lead that is committed to the success of the project will always do better. And yes, this technical lead must have the authority to limit the scope of the system rewrite. Endless scope creep is a recipe for failure. Outside consulting firms will never say "No" to any new request - it means more business for them - their goals are not aligned with the client.