The mistake obviously was not getting the extension done with Betts. Once that happened they turned to Bogey and Sale.
John Henry just didn't think Betts was going to be worth 12 years and $360 mill. He drew the line too low. It's pretty simple, really.
All that said, it's easier to judge what a mistake it was when we've seen Betts prove it with the Dodgers over an extended period.