Backstab as a Dex move is something I've gone back and forth on. I worry a bit that it makes Dex the One Stat To Rule Them all. It allows ranged attacks, Backstab, and Defy Danger, what else does a Thief need?
On the one hand, I agree that Thief with dagger should be totally playable. On the other hand, damage isn't the only measure of playable. That player can still be backstabing to set up the fighter, who deals the real damage. I'll think on it, but I kind of feel like that's just how it is. I tend to think of a little dagger as less dangerous than a big freaking sword.