One of the first and most obvious rules of being a GM is to meet the needs of ownership. That means making the right decisions and following the correct strategy, and executing it in such a way, that ownership's needs are met.
Ben Cherington's overall strategy was geared towards a small market team that can afford to be patient and accept rebuilding years. The Boston Red Sox are not a small market team. Winning now matters in this city and it matters with this ownership. He was a poor GM who did not adjust his thinking to the situation or prioritize the needs of the ownership over the needs of the franchise, and that's why he's gone.
BC was replaced because the dignity and pride of the Boston Red Sox franchise collapsed under Ben Cherington. The prestige BC cost us with his overly-passive approach, and worse, his repeated decisions that took on dead money for no gain, took money out of ownership's pocket. In a smaller market BC's strategy would have been a good one, but this is a large market that needs to win to maintain peak profibability, and BC was not meeting ownershi's needs in this era.
The first year of DD it's clear that he has an ownership mandate to restore the prestige of the big league team first and foremost -- so he did. ANd to his credit, unlike Cherington, he has done a good job so far of getting the right man, sometimes, especially for big market franchises, that's a lot more important than avoiding an overpay. When a team like the Red Sox or Yankees opens their wallet or empties their reserves, it's more important that the guy they get performs than how much he costs. Especially when the #1 goal is a return to relevance.
Sustainability is only ever the goal when there's something to sustain, after all, and Theo did the same thing to rebuild the roster with a 2 year string of deals when he came in as well (and then did the same again in Chicago).
DD now has 3 years to rebuild the farm system and move from a win-now model to a win-sustainably model. He's one of the more ingenious GM's in the business, I don't know why you guys are so unalterably convinced he won't manage to make that transition.