It may, as you say, seem "obvious" in hindsight, but I disagree that a rebuild was imminent. There was no reason to believe, in 2014, that Cousins was going to become injury-prone, as he has. And Malone has proven, in my opinion, that he can create a winning system around the talents (and limitations) of his best player, if you give him enough time. Not only did Sacramento not give him that time, but he also had to deal with a general manager that was determined, come hell or high water, to have the team run a system that was not compatible with his best player's limitations. Malone understood that, D'Alessandro refused to accept that, hence the quick hook. He was looking for a reason to fire Malone, and Cousins' illness gave him one.
In my opinion, it had less to do with a closing window (because your window is nowhere close to be in danger of closing, when your best player is a 24 year-old All-Star, who's still getting better), and more to do with D'Alessandro and Divac being utterly unwilling to maximize their best player's abilities.