Given that some Big Airlines have made the decision that preventive maintenance/replacing items lifed out is too expensive and now operate a “run it until it breaks” policy it’s no wonder there are ADDs. Combined with outsourcing spare inventories as having things to hand is too expensive for the engineering budget.
Of course the fact the ops budget takes a colossal hit for compensation and ad hoc charter is neither here nor there to those who’ve banked their massive cost saving initiatives and moved on! That doesn’t even begin to cover the safety implications of deferring fairly major system components where both are over their life and of similar senescence.