Also in hindsight, it doesn't really impact the player experience of the game.
So you save thaniel which lifts the shadow curse? Except you never get to see that happen. Its only referenced in a cutscene or such once you get into act 3, and then you can't even go back in the game to see the shadow curse removed.
I think this becomes clear when you consider that only two recurring non-companion NPCs get emotionally invested character arcs. Not a terrible thing. I never gave a shit about Esbern or Delphine or the Greybeards in Skyrim. I never gave a shit about Kirrahe or the Quarian Admirals in Mass Effect. I never gave a shit about Tarquin or any of the other boat companions in DOS2. The only two characters who actually changed over the course of BG3 AND had emotional investment in their plots were Barcus, and, oddly enough, Rolan (this one surprised me A LOT in Acts 2-3 as someone who thought very little of Act 1 Rolan).
If Thaniel were a companion, emotional investment and character development would be a necessity. But Thaniel is, at best, a background character. A plot device. A glorified MacGuffin given a few lines of dialogue and a character model. He could've been a glowing orb without a personality and it wouldn't make a difference to the main story. His primary purpose is to give Halsin a quest and character development and to restore the land after you leave.
Unless a character is a member of your party, they are just plot devices, as opposed to complex characters. When you consider that the game needs to continue regardless of whether they are alive or not, their lack of characterization becomes clear.
