Luffy can pump back into g5 mode and didn't turn into old man against Kaido even after defeating Kaido but somehow egghead G5 Luffy can't do that.....just like G4, FS, CoC....with new arc, Luffy do not progress in power but regress in power whereas other characters do progress like Law who is not that much proficient in haki was able to nullify DF effects....
Feels like Oda is doing things random and lazy....if there is any, he needs some operation for his brain too...or some editor should man up and point out his inconsistencies