I would definitely disagree on the Sandoval/Castillo assessment. Sandoval did nothing in Boston to show he was a Major League talent, which is why he was released with a ton of money left on his deal. Castillo did have some offensive struggles related to injury, but also showed himself to be more than capable defensively (which many metrics agreed with). Of course sometimes his injuries were sustained while performing this defense. (Also, Sandoval was NOT a 5/10 player, meeting neither of the two requirements. He was just awful.)
The lineup was etched in stone, but the bullpen wasn’t. Even you admit it was a gamble to rely on Barnes and Brasier. Brasier, who had all of 33IP in the past 6 seasons combined, was a much bigger gamble. As was DD’s longtime man-crush Colten Brewer, who is showing that these gambles don’t always pay off, at least not immediately.
DD has a long history of “winging it” with bullpens. He has had success stories but has also been repeatedly criticized in Detroit for this exact reason. But history has shown, when he needs to save money, the first thing he cuts is the bullpen.