Luffy did beat Kaido though. The guy didn't just hop in the magma just cause, he had his flame dragon punched, horn broken and was hurled so far into the ground he reached magma. I agree, it's disappointing he didn't beat Big Mom after all their issues, I would've also liked to see him whoop Jack but what happened was fitting enough.
I feel like some promises though like Luffy whooping Jack or Zoro rounding up the Wano swordsmen shouldn't be taken too far, sometimes stuff simply doesn't go a certain character's way and they got to make due with the current situation and make compromises.