While this is true, the problem with planet side defences is a simple one. They're fixed targets on a planet with a fixed rotation and orbit. With enough math, you could fire nukes from Pluto, and hit New York in total safety, since you'd be able to see the return fire DAYS or even WEEKS before it gets to you. Just a lazy drift to the left (Yeah, casue we got moves) and you're out of danger
1. While said defenses are fixed, they are also down in the extreme clutter of a planetary surface. Unless it's on, like, Mars or something where there's not a whole lot there and everything is likely to be concentrated in a few hab domes.
2. Since said defenses are down in the extreme clutter of a planetary surface, it is insanely difficult to actually find them to blow them up. Furthermore, since said defenses are on a planet, they intrinsically have much greater firepower and durability than your spaceships ever could. A laser can use the bedrock it's built on as a heatsink, and missile launchers are far easier to supply on a planet's surface than from space, even with space-based manufacturing.
3. In the same way that you can see return fire stupidly long durations of time beforehand, so can New York see your shots from Pluto. This also means they have stupidly long times to shoot down or intercept said ordnance, thus making it a moot point that you can attack from the relative safety of Pluto.
Also keep in mind, if the forces at the near-Sun portion of the Solar System have a giant beamed-power installation, they can use that as a GIANT LAZ0R CANNON!! and then scream "IMAH FIRIN' MAH LAZ0R!1!!" and blow you out of the sky. And you will have to respond with missile weapons, which can be shot down by said beamed-power installation.
4. Even a "lazy drift to the left" isn't going to save you against a kilometers-wide beam of hard radiation. You'll get caught anyway.
So your analysis is a rather flawed one. Yes, surface-to-space defenses are stationary........most of the time. Because if you can build spaceships of tonnage and/or volume comparable to, say, a ballistic missile sub, and put powerful lasers capable of killing surface targets through the dense atmosphere of Earth, then you can put the same lasers into a submarine.
Which can move around for a good while. And submerge into the ocean, where it will be safe from return fire.
Missile silos don't even have to be used; a SCUD could reach an orbital target, and, while it needs a pretty big truck, it still only needs a truck. And a truck is cheaper than a spaceship. By a lot.