Funny because in case of Kaido it actually might be exactly that, about power levels.
He desired death and nobody was strong enough to kill him.
Now that he apparently died, he could Awaken.
And return from dead, like Rupee did.
Probably for same reason why Stussy is named Stussy and not Bakkin - to hide their true identity.
Probably Vegapunk did a lot of it under the radar, cloning the strongest people and DFs to oppose the World Government.
His deal with Kuma is also a mystery.
Yes, it has its own problems but seems that's the route we are going.
We can only hope that a non-clone delivers more but with people like Imu, I dont know if we can expect that.
Rocks meanwhile was a captain of all the great pirates that shaped the era and Roger's greatest competition when it comes to fame and ambitions. He can absolutely deliver as end boss and the groundwork for his cloning has been laid down since Impel Down scene.
I am betting on either Rocks or Garp clones. However, bringing back the lost brother is a solid option too.
But I hope not and Ace is gone forever. Personal bias against Ace tho.