I will tell you why it's not really plot armor/bullshit. He got one-shotted because of his recklessness and weak defense. We know that observation Haki doesn't work well when the user is enraged and not calm. furthermore, If you look at stage 1 advanced armament Haki (emission), you can see how it can be used well enough for defense (Admirals stopping the Gura with it/Sentomaru claiming to have the tightest defense) so I'm willing to bet Luffy can use emission Haki to block Kaido's Kanabo attacks or even shatter it. look at how powerful his emission KKG in Udon was, he casually pulled it out.
You're right, Luffy is much weaker than Kaido physically which further proves that emission/internal destruction which defend/negate Kaido's attack/durability, is the key to cover that massive gap in strength. Although he was physically stronger than Crocodile, Luffy was never meant to be physically stronger than Kaido at all. iirc Oda said during an interview that he doesn't want "Luffy's punch being so strong" to be the method of taking Kaido, rather him emitting his Haki and transferring it into his target which is a more clever way.
You're right, Luffy is much weaker than Kaido physically which further proves that emission/internal destruction which defend/negate Kaido's attack/durability, is the key to cover that massive gap in strength. Although he was physically stronger than Crocodile, Luffy was never meant to be physically stronger than Kaido at all. iirc Oda said during an interview that he doesn't want "Luffy's punch being so strong" to be the method of taking Kaido, rather him emitting his Haki and transferring it into his target which is a more clever way.
Its too much for just a bunch of rookie pirates to take down, the marines will get involved. As well as blackbeard, oda didnt have moriah randomly end up on his island for no reason. Im willing to bet some warlords are coming too( namely hancock and weevil who will follow marco who is sure to show up). This entire saga is called the versus yonko saga, and what better way to end it than having all the old yonko fall. So i think maybe even shanks will show up.
All in all, while luffy made some good progress, i just dont think that he is ready to take down a top tier decisively even with help. Let alone a yonko. And possibly the strongest one at that.