All For One's plan regarding Overhaul's Quirk, as revealed by the vestige, seems illogical and flawed for several reasons:
Expectation vs Reality of Quirk Acquisition:
One would expect All For One to take the entire Overhaul Quirk because it combines high destructive power with the unique ability to disassemble and reassemble matter in any way, which could help him overcome his bodily limitations for storing Quirks.
Instead, All For One has Dr. Garaki remove the reassembly ability, leaving only the destructive Decay function. This Quirk is then handed to Tomura Shigaraki. The removal of the reassembly power significantly diminishes the potential benefits that All For One could have gained.
Missed Opportunities:
Even after escaping Tartarus, All For One could have reclaimed the original Overhaul Quirk from the weakened Chisaki but does not do so. He also allows Chisaki to leave with Lady Nagant, missing another chance to assert control or benefit from the Quirk.
Over-reliance on Tomura Shigaraki:
The entire grand plan hinges on Tomura, an unstable and volatile beneficiary, with All For One gambling that Tomura will be a suitable and strong vessel to inherit One For All.
All For One does not seem to factor in Tomura's unpredictability, which ultimately leads to All For One losing to All Might, ending up crippled and near death.
By placing overwhelming power in the hands of an unstable individual, All For One's carefully laid plans unravel.
Underlying Reasons Explaining This Strategy:
All For One was likely heavily committed to the plan involving Tomura, having invested years into grooming him as a successor.
The destructive part of the Overhaul Quirk overlaps somewhat with Tomura's Decay Quirk, making the full acquisition of Overhaul potentially redundant.
All For One has a known disdain for Quirks with complex learning curves, which Overhaul's Quirk likely entails, especially the reassembly ability.
Attempting to capture and master Overhaul could have posed too great a risk to All For One's fragile state and long-term plans.
He may have preferred to keep the plan simple and focused on Tomura rather than complicating matters with new variables.
Resulting Consequences:
All For One loses the final confrontation and is left incapacitated.
His plan to steal One For All fails due to overdependence on Tomura's choices and his unstable nature.
In conclusion, while the plan appears moronic because it overlooks the massive potential of the complete Overhaul Quirk, it reflects All For One's calculated, albeit flawed, risk assessment and commitment to his chosen successor. His arrogance and single-minded investment in Tomura blinds him to other solutions that could have potentially secured his success better. This strategic rigidity and underestimation of Tomura's instability ultimately cause his downfall. Thus, the plan was fundamentally flawed in its core assumptions and execution, leading to a catastrophic failure. This aligns with in-universe explanations that All For One avoided Overhaul's Quirk due to complexity, risk, and narrative necessity to maintain his weakness and motivation in the story