Very true, however. Two emporer clashing and or uniting, especially with the undertone of the rocks pirates could throw the power balance way off. It seems like the Marines and wg just cant win here
Winning is a relative concept here. When WB died it was pretty much the end of the WB Pirates and the exact same scenario was mentioned by the BM Pirates in the case BM would disappear. Despite the elaborate lengths they went to establish their territory, in the end it all stands or falls with the existence of a single individual: a Yonkou. So when they make a move, it's pretty much all or nothing every time.
The WG on the other hand is a behemoth of an organization that has existed for centuries. For them entities like the admirals are a precious commodity, not something they are going to risk too easily at the expense of their overall strength. Simply put time is pretty much always one the side of the WG as they can wait till a Yonkou implodes.
I only see two reasons as why the WG would even bother with Yonkou:
1) To assert their dominance from time to time. It isn't really good for their credibility if they let them do as they please all the time.
2) To avoid having the OP being discovered.
Honestly I wonder if Ace hadn't been Roger's son would they really have bothered picking a fight with WB? He was old and hadn't done anything noteworthy in years.
So for the WG it's always a careful calculation whether the pro's outweigh the contra's. When one Yonkou makes a move, it causes a domino effect with unpredictable results. Not in the least because of the erratic nature of the emperors. Last time it resulted in one emperor being replaced by a far more devious one and the Great Age of Piracy obtained its second wind.