Assuming this Kaido/Big Mom alliance will last, who is strong enough to actually be a threat to them? Luffy is supposed to be the strongest one in his alliance and he got KO'd in one hit by Kaido. The only ones we know right now that are strong enough to fight them are Garp and the Marines. The Shichibukai have never been strong enough to be a threat to the Yonko, they were there to keep weaker pirates in check, we can talk all day about Mihawk facing Shanks but the Yonko are named as such because of their Power base not because of how good they are in a one on one fight. The only thing keeping the Yonko from taking over the world are the other Yonko and the Admirals, along with the Admiral level Marines like Garp, Sengoku, Kong, Momousagi and Chaton.