I guess the idea is that in the Blood War Karlach was superpowered in some way by Zariel or just being in Avernus, which she no longer will be once she deserts? I'll admit I think we have too many companions/origins who were powerful and no longer are for one reason or another (as long as it is one reason or another - I'll be even more disappointed if it turns out everyone is just nerfed by the tadpoles). We now have Karlach, Gale and Wyll in that position, plus presumably Minsc and Jaheira (going from the comics, Minsc or his copy reappeared at a lower level when his statue was awakened, I can't imagine what the rationale will be for her).
I can see the temptation to structure the stories this way, given that for both companions and player characters I do want an interesting back-story to uncover and resolve, and if the character has lived an interesting enough life to make for a thrilling yarn it then starts getting hard to explain why they never got above level 1. But still, the writers have given themselves some extra work to do to make these stories genuinely distinct.
I do think that as companions, at least, this can be partly addressed by having us meet characters at higher levels as was mentioned above, making the problem at least a bit less obvious when we're not playing multiple origins in a game, though it's going to need more than that to really convince me this was the right way to go.
