With the exception of the World Government vs Whitebeard fight, no one has been able to defeat a Yonko thus far.
Even though Whitebeard was defeated, he was very old and sick, and it STILL took a lot to kill him. Whitebeard was finished off by Marshall D. Teach who, soon thereafter, became a Yonko himself. In short, I believe the only ones strong enough to defeat a Yonko are other Yonkos. As we know Big Mom is on her way to Wano. I think Big Mom and Kaido are either going defeat each other at the same time or they are going to weaken each other to the point that Luffy and the Alliance are going to finish them off.
There will be a lot of powerful villains in Wano: Kaido, Big Mom, Orochi, Kaido’s 3 Disasters, Big Mom’s Sweet Commanders, and the rest of their respective armies.
Currently, Luffy is (arguably) the strongest person in the Alliance and he just got knocked out in one hit.
He needs a power-up. I believe that the prisoner mine is serving as training for Luffy to become exponentially stronger. All he’s doing all day is lifting giant heavy stones and racing to move them to transport ships, eating a ton of carbs and sleeping – all while wearing seastone handcuffs!
This reminds me of Goku training under increased gravity and coming out way stronger. Even though it’s not exactly the same thing, Luffy is still training with a huge handicap. This makes it believable that Luffy will be able to stand up to a Yonko shortly after losing to one.
Eustass Kid is going through the same training / punishment, so he’ll probably get the same power-up as Luffy. Now, it’s pretty obvious that they’ll team up to defeat Kaidou.
So therefore, Big Mom and Kaido will fight each other at some point, they will either defeat each other or weaken each other enough for Luffy and the Alliance to defeat them and finally Luffy and Kid will emerge from the prisoner mines vastly stronger than before.
*Theory by Opeace