At this point, it's been argued to death, and rightfully so, that the main problem with the Star Wars sequel trilogy is that the writing is inconsistent between movies and there was no overarching plan for the story, which leads to a lot of plot threads getting introduced and then dropped by the next filmmaker. Obviously, each film in the series should be its own story and perhaps develop themes that the others didn't touch on, but the stories should still be somewhat overarching and fit in with each other. Otherwise, certain ideas get introduced in one movie and then never discussed again.
This is exactly what happened with Finn's stormtrooper storyline. In general, a lot of elements of Finn's story and character got dropped after The Force Awakens, which makes him feel like a somewhat two-dimensional character. The widely-circulated joke about him doing nothing in most of the sequel movies except for screaming Rey's name is a little too accurate, and it's really strange that these films had the opportunity to explore such a nuanced and interesting concept with the stormtroopers but dropped it almost immediately.
Star Wars could really benefit from exploring new storylines and themes rather than just going back to the old ones and relying on nostalgia all the time. The movies always seem to go back to the same characters and the same ideas around legacy and family and good vs. evil that have been done multiple times before. It's not as though those characters or stories aren't interesting, but it feels like Star Wars keeps going in circles rather than trying to branch out with its themes and try something newer or bolder. The stormtrooper storyline could have been just that if they didn't bail on it the way they did.
In The Force Awakens, the audience is introduced to Finn, a stormtrooper who runs away from the First Order because his goals and views no longer align with theirs, and he has to learn how to be his own person and join the Resistance when he's never been allowed to be an individual before. In fact, he didn't even have a proper name until he gives himself one. This is such an interesting concept that the Star Wars movies hadn't explored, and the idea of learning more about what it means to be a soldier for the bad guys in the way that the stormtroopers are could have been so interesting.
Instead, the series decides to never bring this up again after the first movie (unless it's convenient to the plot, such as when they need Finn to have intimate knowledge of a First Order base so that they can infiltrate it) and the focus of the story moves away from Finn entirely. Yes, Rey is the main character of the sequel trilogy, but it seemed like they were also setting up Finn to be a secondary protagonist in the first movie, only to totally pivot away from that. Not only is that a huge disservice to the character and to John Boyega's performance in that role, but it also means they dropped one of the most interesting plot lines from that first movie.
Stormtroopers are famously anonymous, and for a good reason. Having faceless bad guys for the heroes to fight means that it doesn't make the heroes seem like bad people if they're constantly killing evil cronies. You don't want to see Luke Skywalker as a murderer, and it's much easier to avoid that comparison when the stormtroopers he's mowing down feel like non-human robots, with no feeling or compulsions other than to serve the Empire. However, it's a really interesting thought experiment to consider if the stormtroopers are suddenly fully-fledged people, like Finn. Obviously, he's a special case because he has enough independent thought to break free, but it calls into question how much agency the other stormtroopers have.
If Star Wars wants to keep exploring the "good vs. evil" (or rather, light side vs. dark side) debate, it would have been cool to have the heroes confronted with whether or not killing stormtroopers makes them murderers, now that it's been proven that stormtroopers can have inner lives as well. Sure, it's a war, but that doesn't make the act of killing another human any less traumatic, and yet it's something that the series never grapples with when it comes to stormtroopers or other faceless "bad guy" soldiers.
They also could have expanded Finn's character more with this story, and perhaps gotten into his past traumas as an agent of the First Order, or maybe even explored any sort of brainwashing or propaganda that those stormtroopers might have been subject to in order to keep their allegiances steady. Finn would have had a lot of ideology to unlearn after breaking free of that environment, and it could have been a really poignant allegory for real-life examples of leaving an ideology that one was raised in but no longer believes in, like a religion or an abusive family situation.
But unfortunately, none of this happened, and the storyline was dropped so quickly that it's hard to remember it ever even existed in the first place. It was a storyline with so much potential that was wasted by poor planning, which is honestly a statement that goes for the sequel trilogy as a whole. Maybe it's something that Star Wars could explore in a future project, but it's still so strange that it seems like the sequels were going to tackle the issue but decided to throw it away. If the franchise wants to keep enduring, they're going to have to start examining different themes in this vast universe, and maybe explore topics that they haven't touched on before, rather than rehashing the same conflicts over and over again.