Sure, a million space rocks would be difficult to stop. But to say you have the capability to fire all of them means you surely have the ability to make a lot more standard missiles. If you have a million space rocks (you'd need the rockets to propel them, likely many per space rock), then I'd have several times more explosive-carrying missiles. Stopping your space rocks would seem like much less of an issue, then.
In the scenario you described, you say you "bring in your fleet" as if it were some freebie. If someone had the power to stop a fleet much larger than your own of space rocks, then I'd have to assume they have the power to take on a fleet as well. The protection of Earth from space rocks could occur from the planet's surface itself (or other Sol system planets); no space fleet required. With your rocks gone, you only have your fleet to use, which is now at the mercy of the Earth-borne fleet given you've "brought them in" (plus whatever else Earth's surface has left in anti-spacerock weaponry).