True but the marines also had a huge advantage in the arena. They set up for that defense in advance because they knew a war was coming. The walls being deployed is a good exampe, because of those walls a large portion of WB's forces were killed forcing WB to take a primary role ultimately causing his death. Had he been able to hang back, things might have turned out somewhat differently. I do agree though that the marines did beat them back pretty well though.
But unless the marines are hiding a force equal to that at marineford in the new world. There is no way they can take on all the yonkou at once. Each Yonkou themselves could fight an admiral and with their first mates/calamities etc they could beat the admirals. Without doubt the marines have less numbers than the yonkous combined too. They are pretty outmatched in firepower, they haven't lost control though because non of the yonkou want the others to gain power. The marines are also not alone in their effort. CP helps and so does the shichibukai.
If the yonkou could be defeated individually by the navy, why havent they done so? And better yet, if the marines could take on all the yonkou and win, why havent they done so? Answer: because the marines cant, with the WG and CP and the shichibukai it has turned into a big stalemate. So how havent they been beaten yet if the marines could do it by themselves let alone with the help of the WG shichibukai and CP?