Tbf, I dont think thats been their goal. The balance of power between Marines, Shichibukai and Yonko, imo, was made so that way they can:
1. Paint pirates as evil (because some of them are)
2. Make themselves look like saints
3. Prevent others from revolting by believing in their cause.
So its necessarily that they just could have gotten up and crushed the opposition, its that they are trying to maintain the illusion of power full stop. This is actually WHY i think Shanks is fishy, he potentially maintains "peace" for them so that way the balance stays the equal. Though obviously he has some ulterior motives with Luffy which could be good or bad anyway.
Anyway, this only really changed once York gave them a fuel source that allows them to power their weapon to destroy full islands. Now they can obliterate opposition without anyone even knowing (except of course Sabo surviving lmao).
You can see here that at the Reverie is when the Gorosei decide to start talking about another "Great Cleansing" again, even including the fact that the Paramount War is factored into the decision, something Shanks tried to prevent. They specifically state that the worlds equilibrium cannot be maintained forever:
Tldr: The Gorosei not doing anything is by design, they were trying to maintain a balance of power to stay in true control