The reason why a lot of the pieces are already there is because Karlachs quest already existed in some form and was then cut from the game as the quest happened in the upper city that got removed.
Yeah that's seems highly probable. I mean look at the official Upper city map presented by Larian. Everything is here, Steel Watcher Foundry included. And there is a quest icon on the map, meaning there is at least one quest here that is, by force of circumstances, cut from the actual release.
HYPOTHESIS TIME :
The more I think about it, the more I'm conviced it's already done, quests are developed, story is already written and dialogues are recorded. And we'll have the rest of the storyline for companions that are related to Baldur's Gate like Astarion and Karlach.
But with the Starfield release and their panic PC release announce a month sooner, I think they have cut maybe half of the Act 3, thinking most of the people will be able to wait a month, or that it will took for a majority of the players more than a month to finish Act 1&2. Then for the PS5 release we should have the full game with probably a lot of bugs fixed.
I'm pretty sure that with all the quests and the story related to Upper City, Karlach, Astarion, Gortash and all the final antagonists will take on much more depth and scope in the story. This could explain among other things, why actual differents ending feel bland. Maybe these will be triggerable endings earlier in the full story, which necessarily takes away a lot of epicness.
And this could be a new concept invented by Larian : the Early Full Release Access.
