I understand that sports in general and specifically baseball in this case is often a matter of "What Have You Done For Me LATELY" but IMO firing DD for constructing a WSC team in 2018 and having the team fail in 2019 would have been reactionary, especially if JH had set hard spending limits.
That's why I'm tempted to believe that there were three things that entered into the firing. The first was the failure of the 2019 season, the second was that DD's history said that he wasn't a guy with the skills to rebuild, and the third is that DD had just plain "worn out his welcome" in Boston.
If what we read is true he lacked the interpersonal skills to maintain a relationship with others in the FO. (IOW, he was a prick and JH got tired of him!). However, I'm also aware that nobody escapes from the Red Sox FO without some kind of character assassination, so I take all of that with a grain of salt.
Consequently I'm tempted to believe that it was both of the first two reasons I mentioned above, but as Bell said, we'll never know for sure.
I do have to wonder though... do other organizations trash people on their way out like the Sox do, and if not wouldn't that have some effect on who might or might not want to come to Boston?