Whitebeard was on life support even before going to Marineford and suffered a heart attack in the heat of battle. I hardly call the marines strong when they can barely squeeze out a victory against a literal dying old man. Replace Whitebeard with Shanks, Linlin, Kaido or potentially current Blackbeard and the landscape is night and day different. You'd need 2 Admirals to stall a Yonko and that frees up the commanders to gang rape whoever's left out.
Also, a reminder that a number of special circumstances combined such as a slew of powerful Impel Down prisoners, two Warlords outright betraying the Marines, another Warlord undermining the Marines, Luffy and the arrival of Shanks, are the only reason the WB pirates were not slaughtered to a man.