I think you're writing Poirier off a little bit too soon. There are some fighters that haven't been the same after a knockout, but Poirier has been KO'ed in a past (by Johnson or McGregor) but managed to bounce back nicely. There's no reason to think it'll be much different after KO from Gaethje (who also got KO'ed and that didn't stop his way to the top).
Usually fighters will be a little different after a knockout and that happens a lot among fighters. Regarding Dustin, I can't say whether it will affect his performance in the future or not because Justin's knockout defeat yesterday was the most brutal knockout Dustin experienced. A fighter will remember that moment a little when he is in the same situation in the next fight.