sorta been sitting on this thought since the lightfall release trailer came out, but i think its a frankly very weird to introduce an entirely new location and previously unknown group of people in your second to last dlc (i also think calus becoming the main big bad is...an odd choice, he's not their strongest villain, but thats another thing) but i also think its a good example of how seasonal storytelling + the idea of constant content changes storytelling in really odd ways.

when you CONSTANTLY have to be generating content, you need as many threads hanging around as possible so you can pick up one of them and make a season about it when you need it (or a raid, or a dungeon, or a strike, or an exotic weapon quest, etc etc). introducing a new location introduces new avenues for story development in different directions, just in case you need them in the future, instead of using the expansion to close an already existing story. this is how storytelling in destiny has been going essentially since the seasonal structure was introduced, and it's why we have so many loose plot threads that have been waiting to go somewhere for ages. i think it's in part the reason why ghaul, panoptes, and xol are all permakilled in their respective arcs, but the lunar pyramid, eramis, and savathun are all left on the table at the end of theirs - it's so that writers can do exactly what they're doing now with eramis, and pull those characters into a seasonal storyline when they need someone to be a villain or a focus (and the leviathan mysteriously dissapearing and returning in haunted). we know that things like ana and rasputin, osiris' coma, xivu, savathun's conclusion + whatever's up with immaru, are all probably going to be resolved in seasons eventually - but the reason that they aren't resolved yet is because of the seasonal model, and when they are resolved we're gonna get a bunch of new dangling threads again becuase they need options for what to turn to in the next content they inevitably have to create, which is the function of things like neomuna + the cloudstriders or rhulk and the concept of disciples

and in general thats fine, having unanswered questions and unfinished stories is completely fine!! but when they are so clearly headed into the endgame, and this content delivery method means that instead of actually preparing for the endgame in any meaningful way, they have to keep introducing new things, it's a problem and makes for a confusing narrative.

and to be completely clear, this isn't a judgement on the destiny writers at all - it seems to me that they're making the best of a bad situation, and they're still doing some completely phenomenal writing. it's an issue with how content creatation and consumption works in the game industry as a whole. it's a huge issue that aaa games are moving towards the model of constant content and constant updates, because it drives storytelling to a very very weird place, when storytelling in games is already an issue