Not necessarily. Let's not forget Shiryu's hint suggesting the last road poneglyph to be linked with the world government, so it's not impossible that they'd be dragged into the battle too.
And ultimately the decisive plot is always D vs Celestial Dragons, with the marines being their outward face, so I could see a scenario where their outward face(marines) is dragged into the battle, and destroyed, dragging the celestial dragons from their perch(like implied by Doflamingo), with the potential discover of the gorosei or IMU's fighting power
After all the world government is an organization that existed long before even the marines came to play, with a hidden story since Joyboy, so it shouldn't be surprising if they actually had capable fighters either through the gorosei or IMU himself