Apparently DOs weren't really necessary, so Watchtower abolished them.
But I think the Society will suffer from unintended consequences by abolishing district overseers. DOs didn't really do anything much useful in and of themselves. But I do think the position of DO performed one incredibly important function: if offered all circuit overseers the prospect of promotion to an easier and more prestigious role in the organisation. Even if most COs never became DOs, the idea that one day they might was probably a good incentive for many DOs. With the role of DO abolished there is really not much if any scope for COs to get "promoted" in the organisation, I think this will make the position of CO far less attractive and can't be good for morale.