It didnt even need to be happy ending imo, you can imagine that engine fix could be hardly achieveable, putting some other NPCs or companions in danger ie during Dammon and Gondians fixing it due to some mechanical unexpected failure hellfire burst would burn one of them to ash. Or the fix meaning you're cut off or conflicting with good ending of other companion due to time constraints on the quests or whatever. Or new engine being not fully compatible causing physical limitations etc. You can imagine plenty of ways where its an additional ending with some offseting sacrifice within the theme of character conflict and MC conflict. It could add value to the tragedy since it would be fully in player hands with all the consequences following the choice.
While every companion has a questline that logically progresses thru the acts and serves their personal story closure (with MC having potential input to their stories in multiple places), Karlach's questline starts this way, but immediately in act2 is cut short and then feels like it serves the arbitraly decided end game state endings rather than herself (ignoring the logical possibilities and creating plotholes along the way).
