In a war all sides suffer losses and obviously all sides prefer peaceful ways to avoid those losses.
But for WG having warlords+ yonkou system was basically because to have balance of power.
They cherish it so much that they immediately made Luffy a yonkou the moment kaido lost.
And, with all the resources WG has - Admirals, VA with potential VA who can be admirals+ FA + Seraphims + former navy veterans like Garp and Sengoku+ Gorosoi themselves and millions of soliers -----no yonkou crew is a match
Heck, whole purpose of luffy making allies and Grandfleet all over his journey is because he will be needing it to overcome WG
Admirals- One is disobedient and probably not even willing to really be a Marine anymore
One got cooked by Momo
Kizaru is the only loyal/competent one
Akainu is an asshole
Vice Admirals-
One got smoked by Law once Law got his heart back
One would have gotten neg diffed by Sabo if Fujitora wasn't there to save them
One is the biggest fucking jobber of all time, even more than JACK AND ULTI, and lost to:
Luffy
Boa Hancock
Law
Vergo
and Doflamingo.
Garp is the only one that doesn't get destroyed by everyone and their mother, but he's clearly not too loyal nor does he like their bull shit because he was going to murder Akainu if not for Sengoku.
Others just sit around and talk to each other all day.
The Seraphim are valid, they're kind of strong honestly but they'd probably get smoked by the likes of Commanders lol.
The Gorosei.. Fuck I'm not writing all of this out just check this great Reddit post about what a mess the Marines are:
https://www.reddit.com/r/OnePiece/comments/w8a7ba