I feel giving the smd a toggle to shoot down a novax is reasonable. The missiles only cost like 3.6k vs the satellites 10k to replace. If its something you can turn on then you can't use them to overwhelm the smd either and since its not economic to build smds all over the map for most of the game it protects the core base and still lets the extent mass be threatened. The sat can do its job to harass outer mexes, defend against pushes and even be an early scout.
Addendum: If needed you could even buff the sat to compensate, drop its reload from 20 secs to 16 to make it more useful as a defensive unit or to kill far units and buildings. this would raise its dps to ~300ish.