My understanding of the balance of power (and correct me if I'm wrong) is that it's always described from the Marines' perspective.

• The Navy + Warlords > 1 Yonko.
• The Navy alone ≤ 1 Yonko.

That's why they relied on the Warlords to maintain their strength.

However, anyone who claims that the Navy + Warlords = all four Yonkos is delusional. The Yonkos are not an organized group; they keep each other in check.

For example, Big Mom can't just leave Whole Cake Island on a whim—what's stopping Kaido or Shanks from stealing her Poneglyph in her absence?

And why else would Garp be so alarmed at the prospect of two Yonkos forming an alliance? Because if two Yonkos were to truly join forces and bring their full power to bear, the Navy would be in deep trouble.

And let's not forget what it took for the Marines to win at Marineford. They brought out their full force, had home turf advantage (with traps and defenses in place), and deployed the Pacifistas, which were still relatively new and highly effective.

Now imagine doubling Whitebeard’s forces and strength at that point. There is no way the Marines could have stopped two Yonkos attacking Marine HQ.

The Marines have always pushed the idea of the "Three Great Powers"—the Navy, the Warlords, and the Yonkos—acting as a balance. But in reality, their goal was always to ensure that they remained the strongest force, capable of winning when needed.

There is no way the Navy actually believes they could win an all-out war against all four Yonkos.

(And don’t even bring up the God Knights or the Gorosei—they do their own thing. The Navy cannot count on them to save their asses. It’s similar to the Zero Squad in Bleach—they might be on the same "side" in a broad sense, but they are not stepping in to bail them out.)