Their never crushing the Yonkos until the MF was actually rather simple, imo: The Yonko system is practically an organized crime syndicate and makes it easy to track down threats.
As we saw from WB dying, a damage to that system means a global-level chaos and thus maintaining it is the lesser of two evils for the Marines.
As we saw from WB dying, a damage to that system means a global-level chaos and thus maintaining it is the lesser of two evils for the Marines.