It's not just about code. In any non-trivial system you have a bunch of components, and any minor occurrence of "bad design" that affects boundaries, protocols, configuration, deployment, persistence schema, upgrade procedures, etc, can lead to huge complications down the road if there's no one to catch this early. Mediocre engineers will only make it worse and worse, and you'll get something super complex that costs big bucks to maintain. If you have someone capable of detecting & fixing such bad patterns early enough, in extreme cases that person can be considered a 1000x engineer.