WB requires that plus more for the one sided victory. Why would the marines only use a small chunk of their forces and risk losing them all? They always gun for overkill. It's no different as to how Kaido freaked out about BM fearing a war, when he had the stronger force. It's all about resource management
The Marines and WG are the ones who care about the balance.
It turned out favorably for them, but the marines were under no illusions that they could lose Marineford. With all everything they'd prepared, victory was still not guaranteed against
ONE of the four emperors, and the marines weren't able to seriously focus on anything else while doing battle with Whitebeard.
Even though the results were still favourable to them, it wasn't overkill. They needed that many forces to stand as good a chance as possible at victory.
The marines literally gathered everything they had to fight one of the four emperors and their victory was barely certain, even with home ground advantage(which they used very well). If the marines didn't fight in their home turf, things likely would have gone a whole lot worse.
Nothing suggests that the Marines have enough forces to take on ALL 4 EMPERORS AT THE SAME TIME even if they did have the warlords.
If all four emperors teamed up to fight the marines and the warlords, the marines would be wiped out.
The marines couldn't juggle capturing Rayliegh and fighting the Whitebeard pirates, so I have no idea how the hell they'd be able to manage fighting all of them at the same times.
The reason this doesn't happen, is because the emperors are fighting amongst each other just as much as they fight with the marines.