I personally disagree with the sequel-bait theory for the ending.
The ending is anti-climactic, but it fits in with the show's philosophy and outcomes in a realistic way. Akane does not take down the Sybil system due to Kogami getting his way, avoiding the government and exploring more of his philosophical side in solitude for the rest of his life. Akane does vow to take down the system, but it's a long, arduous process to do so from within, so something like this takes time. Thus the cycle of Inspectors/Enforcers continues, with Akane continuing to do her job while trying to figure out a way to change things slowly from within. The ending is uncathartic to a degree because it shows the inevitability of everything to change without having to necessarily tell us straight-up. It alludes to these possibilities subtextually throughout the whole show. Therefore, a sequel wouldn't be necessary, as the loose ends are tied up and while one MAY be able to explore the possibilities in the future, one could get a good idea that it's going to take some time.
Kinda daring and ballsy to have an ending like this in this day and age, but the uncathartic nature of it works.