The question is how much of his success is Justin Jefferson, Jordan Addison, KOC and a decent o line. He has maybe the best receiver duo and a top 5 offensive play caller.
If Darnold leaves I would bet big he flops at his next destination.
He doesn't need to be an upgrade, he just needs to be reasonably close. If we can get 80% of Darnold it is probably a wash with the 30 million we save.