EDIT: Ah, I found that he did indeed decide to centralize the militias a day before he died.

He then met with Cipriano Mera, who proposed that all the confederal militias in Madrid be unified under Durruti’s single command; this would prevent an army from being formed, while also relinquishing the democratic control the rank-and-file had over the command structure. Mera and Durruti then agreed to meet the following morning

[…] There he agreed to Mera’s proposal that they join their forces under his command and enforce strict discipline, but he remained immediately preoccupied with relieving his men as soon as possible, and told Mera they would revisit the issue in their meeting with Val later that day.

I’m surprised he agreed to go that route, honestly.

End of edit.

Could you share your source which details that Durruti created specifically a top-down centralized militia? From the sources I’ve read, he created a bottom-up militia with the ability to recall poorly performing elected leaders. As an example, from Chapter 7 of Paz Abel’s ‘Durruti in The Spanish Revolution’:

The volunteers decided among themselves how to organize themselves, and all opposed anything that suggested a resuscitation of the militarist spirit or hierarchies of command. The structure and organization of the militias, which lasted until the general militarization in March 1937, emerged from the discussions among the future combatants. It was simple: ten men constituted a group, which nominated a representative; ten groups formed a centuria, which elected a representative of its own; and five centuries would form an agrupación. The leader of the agrupación and the centuria delegates made up the agrupación committee. [540]

Pérez Farràs, the Durruti Column’s first military advisor, objected to this organizational structure and cast doubts about its feasibility in combat. Durruti quickly realized that Pérez Farràs would not make a good advisor and replaced him with artillery Sergeant Manzana, who had a better grasp of the anarchists’ anti-authoritarian psychology. Durruti entrusted Manzana and Carreño (a school teacher) with equipping the Column with artillery, munitions, as well as doctors, nurses, and an emergency operating room. Manzana didn’t need many explanations. He immediately understood what Durruti wanted from him and did a wonderful job carrying out his mission. He knew several soldiers who had joined the column, as well as some officers, and planned to have the military men instruct the others. All these people integrated themselves into the Column, fraternally and without conflict.

One day Pérez Farràs stated his criticisms to Durruti directly: “You can’t fight like that,” he declared. In reply, Durruti said:

I’ve said it once and I’ll say it again: I’ve been an anarchist my whole life and the fact that I’m responsible for this human collectivity won’t change my convictions. It was as an anarchist that I agreed to carry out the task that the Central Committee of Anti-Fascist Militias entrusted to me.

I don’t believe—and everything happening around us confirms this— that you can run a workers’ militia according to classical military rules. I believe that discipline, coordination, and planning are indispensable, but we shouldn’t define them in the terms of the world that we’re destroying. We have to build on new foundations. My comrades and I are convinced that solidarity is the best incentive for arousing individual responsibility and a willingness to accept discipline as an act of self-discipline.

War has been imposed upon us and this battle will be different than those we’ve fought in Barcelona, but our goal is revolutionary victory. This means defeating the enemy, but also a radical change in men. For that change to occur, man must learn to live and conduct himself as a free man, an apprenticeship that develops his personality and sense of responsibility, his capacity to be master of his own acts. The worker on the job not only transforms the material on which he works, but also transforms himself through that work. The combatant is nothing more than a worker whose tool is a rifle—and he should strive toward the same objective as the worker. One can’t behave like an obedient soldier, but as a conscious man who understands the importance of what he’s doing. I know that it’s not easy to achieve this, but I also know that what can’t be accomplished with reason will not be obtained by force. If we have to sustain our military apparatus with fear, then we won’t have changed anything except the color of the fear. It’s only by freeing itself from fear that society can build itself in freedom.[541]

Durruti had expressed himself with extreme clarity. His goal was to unite theory and practice. As an anarchist, he intended to remain faithful to libertarian ideals while leading a workers’ column that would soon fight important in Aragón, on the frontlines as well as among the peasants in the rearguard. [542]