I think Darnold could have his best season under KOC. He's bounced around in terrible situations (Jets/Panthers), then backed up Purdy when he actually had a good team around him. If nothing else, the Vikings have a strong OL and some good weapons that Darnold never had in his prior stops as the starter.
Edit: To clarify, I don't think Darnold will be anything special for the Vikings, but I don't think he'll be terrible either.