Whitebeard died before the final arc as well. Imu-sama is nothing more than a shadow about which speculations of great strength exist. The Gorosei, up to this point, have no accomplishments better than Kaido. They literally failed to conquer Wano, a place Kaido ruled for decades, and they fear upsetting him.
Blackbeard may come close to Kaido's level, but his last mention of Kaido was that Kaido was a fearsome monster, and this is considering that Blackbeard already had two Devil Fruits.
It's hard to see these characters surpassing Kaido when they've all praised him before.