Then you would still have 8 ballistae, the same as if you stuck two mediums on a decurion. Assuming you get fifty basileus, that's 400 ballistae every five seconds, more than enough to kill anything. Fourty decurions is 320 missiles. Eleven archers is 132 missiles, but you get FMR to make up for it.
The problem is in the concept of long-range missiles. Get enough of them and you win. End of story.
I can, and probably will, make it a single-fire missile. After all, LRMs are getting regenerating ammo soon. But that doesn't solve the underlying problem. Removing the weapon entirely would not fix it either, since the problem exists with pilums as well.