The more I think of it, the more I doubt a happy ending WOULDN’T take time and resources away from the 4 upcoming DLCs/expansions. If a happy ending means both Noctis and Luna survive, it would mean a complete redo of the game from chapter 9 onwards. Luna doesn’t die, so Noctis and Gladio don’t fight in the next chapter. If Ignis doesn’t go blind in the happy ending, then the entirety of chapter 10 is obsolete, as is every cutscene and scene thereafter where he is blind. With Luna alive, Ravus doesn’t have a motivation for betraying the Empire, so Tenebrae isn’t attacked in retaliation. Noctis doesn’t have his meltdown in the train mourning Luna, there’s no need for the flashbacks in the flower fields, Ravus isn’t killed, Noctis doesn’t have to sacrifice himself so his conversation with Bahamut is unnecessary, and so on, and so forth.
The two other ways I can see this going down are:
Noctis and Luna (and perhaps other characters like Ravus, Jared, Regis, etc.) are REVIVED at the end by means of some mumbo jumbo. But the power of revivification would come completely out of left field given that, until now, it hasn't been part of the in-game lore (at least for humans), and would completely undermine the stakes of the narrative. If you can revive someone, then the story loses all sense of urgency.
Or…
The alternate ending is triggered during chapter 9, basically skipping what’s left of the story (like they did in Episode Ignis) so they don’t have to rework anything... which would be a horrible way to finish the story, basically skipping half of the game (including the entire third act/Tenebrae/Episode Prompto/etc) and finishing the main story at around 10 hours (probably less) or so.