With Big Mom I simply see no narrative satisfaction with having her be defeated here and now by Kid and Law. They are simply the wrong people to do this imo.
With Kaido it's kind of whatever.
I'm not irked that she goes down, but I'm just tired of all these standard shounen tropes of characters tanking damage with no effect on them at all, being able to pull out new powers and trump cards while seemingly on their last leg and so on.
It makes fights feel like nothing really has an impact or consequence. It's just characters showing off their powers until Oda decides "This is the one attack that actually sticks."
I haven't been enjoying One Piece fights in a very long time, fights make no sense anymore, perhaps they never have, this is just Fairy Tail levels of writing of having characters declare their limitation or current state only to prove themselves and everyone else wrong.
Actions don't have consequences anymore, anything goes, so I'm just happy we are finally closing them all down and can move on with the story.