Here's the thing about the Denied's place in the narrative:
I'm actually glad that they included the Denied less so as a central antagonist and more so as precursurs to the Equalists who would pick up where they left off. They're a facet of the post-war world of Avatar, one that Aang has inevitably brushed up against. Like this is a sequel to a movie where they were the central focus.
Additionally, I do like that they don't have their beef resolved so quickly. Their's is an anger that Aang can't easily talk them down from. Sure, his friend got through to one of them but the wider radical group clearly was not going anywhere over pretty words.
Hell, it's why I liked that they never actually showed the Equalists dissolve after Amon's unmasking. No doubt it contributed to them not being out in the open again but that kind of resentment that drove them is likely still in hiding.
Some have been caught while the many others have clearly slunk into the shadows to fight another day. A day when a masked man presents them with a very charismatic backstory and a seemingly permanent solution to their problems.
That's not to say I wouldn't mind other adventures of Aang being show on screen or in books regarding this radical group. Make them like what Magneto was to the X-Men in a way with them never truly going away. That's what I always felt LoK could've done with the Equalists being smaller but sometimes allying themselves with a villain if it means non-benders can reap the rewards.
