The simple answer: they go for Broken Steel route like bethesda did in Fallout 3 because this garbage ending provides continuity for possible avernus dlc. Or maybe some writer got their ego hurt and can't stand that players don't like Karlach being the only character without happy ending, I've seen authors, writers and companies doing much dumber. Also, "hoping for the best while expecting the worst" is not a tactic. That's the coping mechanism, we'll either get what we want or make sure Larian's reputation about listening to players is a tale from the days gone
