No one knows the general ratio of Marines to pirates are in the one piece world....but ehhhhhh...it's surprising that the Marines (in my opinion) are not dedicated enough...if they want to snuff out piracy... kizaru in order to kill time can just take a marine battleship and enter the headline and just deal with all the pirates he encounters in the grand line....I'm assuming that won't take a lot of time.... while the yonkos will remain....the upcoming threats the Marines would have to deal with are minimized... the way i see it....the Marines just enjoy or don't seem to mind the destruction the pirates are causing and like to keep them as a check and balance in the one piece system...if the senior officers do not engage the pirates aggressively enough....a case like luffy will just rise up the ranks to cause trouble...and by then it would become to difficult for them