I don't really understand why this is the case. Nukes don't defend against nukes—MAD defends against WMDs. Why do new and fancier WMDs make MAD no longer effective?
In an arms race there may be a period of technological superiority. During that period MAD does not exist.
Bombardment from space creates at least two scenarios that upset MAD.
1. First strike capability
Small projectiles sufficiently accelerated may escape detection from ballistic detection systems. This could give an actor sufficient confidence in their ability to eliminate return strike capability.
2. Plausible deniability
For a period it may not be possible to determine which actor launched a strike. This can cause sufficient hesitation (and the expectation thereof) for an actor to initiate a strike and the target to fail to retaliate due to insufficient confidence in their target.
I'm sure there are other scenarios as well.
You don't, although mobile launchers are not out of the question. A rock catapult is far easier to construct than a nuclear missile launchpad (given available materials). Notably the munitions are fashioned anywhere on the moon and the munition needs no guidance, other than the initial launch window timing.
Right but I’m not talking about construction. I’m talking about destruction. See an enemy building a catapult with your moon-orbiting spy satellite? Send your Space Force astronauts in a rover to dismantle or destroy it. Or send an unmanned rover with robotic, remote controlled arms when we get to that capability.
Several of these components are necessary even for a peaceful research base.
That's not very effective if you can fire and abandon, since it's very likely that someone could work out the locality of the origin, you have to assume there's no way to defend against that. So what? Another one pops up in a few days.
The only way to defend against kinetic bombardment is controlling material gathering/supply and pinpoint destruction and that won't even be an assured defense. In a year, someone could scrap together X materials to hurl another.
Yeehaa!
Not sure I buy it, but if it were taken seriously it could certainly light a fire under the collective keisters of the world's superpowers.