I mean still... if Luffy can't defeat a Yonko by himself then him being able to defeat Blackbeard who will be at that time even stronger than Roger doesn't make sense. Blackbeard eating multiple DF should pay off, Blackbeard is going to be the strongest in history and for Luffy to be able defeat someone like him means that his power progression should be systematically. Luffy vs Blackbeard is going to be his thoughts fight, tougher than his fight against Kaido otherwise Kaido would have been the antagonist instead of Blackbeard. Blackbeard on top of his overpowered DFs can take away Luffy's DF abilities, the fact that Luffy has been straggling against YCs made people to believe that Luffy can't defeat Kaido, and thus he can't defeat Blackbeard too, and if Luffy will not be able to defeat a Yonko by himself before his fight against Kaido then even I don't see Luffy having 1v1 against Blackbeard. Some people even say it's going to be Luffy and Shanks vs Blackbeard.
OT: Luffy doing even half the work against Kaido is a stretch considering how many people will take Kaido on.
If he does 20% of the job, he can consider himself lucky.