I must admit I found the ending a bit "flat", perhaps because I also found it quite confusing.
If I understood properly, the final comments suggested that the whole endeavour to root out corruption was largely a waste of time since it was mostly covered up.
I don't know if anybody has been watching the three-part series Bent Coppes: Crossing the Line of Duty, the events of which it is reported Line of Duty was loosely based on. It was truly shocking. People at very senior levels in the Met and City of London Police were in league with the criminal groups and, apart from a few token prosecutions, the systemic corruption was largely covered up. Junior police officers were, as it were, inducted into a chain of corrupt officers and it seems some felt they could not take their complaints to anyone since they knew that some of the people at the top were part of the problem. A former woman police officers did apparently "whistleblow" and was more or less sent to coventry by her colleagues. I think police officers of any rank, but particularly those in very senior roles, who engage in these practices are worse than the criminals.