One of the big complaints about Imu, the Gorosei, and Holy Knights being super strong is that it makes the BoP meaningless since the WG could easily wipe out the Yonko if they wanted to. While this is true, the reason why it’s really important to the WG that the Marines and Warlords balance out the Yonko is that the CDs view themselves as actual gods. It would be beneath them to get involved in mere human affairs, so the Marines and Warlords being sufficiently strong is important for making sure that the CDs don’t have to get themselves involved. The CDs would only involve themselves if the matter directly involves them (such as the RA trying to starve out Mariejois) or if it’s an existential threat to the WG