You're talking about boost-phase interceptors, not mid-course interceptors. Starship to replace GMD (as suggested by credit_guy) doesn't make much sense, but Starship does have the technical ability to launch a massive constellation of space-based interceptors for quite cheap. With Starship, it should be possible to put tens of thousands of interceptors into LEO, and to rapidly replenish the constellation.
As for the "general consensus that weapons in space are bad", there's also a general consensus that war is bad, so we should all breath easy! Wait no.
The problem with weapons in space are that they put the opposing super-powers in an impossible position. Hence it forces them to make a move asap to prevent being at a total disadvantage. This is mostly because weapons in space can be used with very little warning. There is little defense, and not even any notice that allows you to prepare or setup a second strike.
Plenty of people don't think that's bad, but it seems that the top military decision makers of all super-powers do see the problem with that, and hence they have managed to keep weapons out of space.
This remains a high-level decision, so it only takes a few rational people for the game-theory to work.
Consider the external politics around mobilization. It is a big deal to mobilize externally, because it forces opposing players to also mobilize, hence moving everyone closer to conflict. The thing about weapons in space is that they are effectively always mobilized. Even if you could sustain perpetual mobilization, it wouldn't be a good strategic move.
Something similar goes for missile defense systems that are too effective. It disrupts MAD, and pushes the opposing side in to a corner. Either make a move before the defense system is up, or accept a role as a second grade power from now on. The problem, specifically, is that it removes the second-strike capability much more than that it removes the first-strike capability.
So having a missile-defense system in orbit has game-theoretical downsides both for being in space, and for removing the enemies second-strike capability.
The same does not quite hold for having the capability to launch such a missile defense system. It gives your opponent time to react when you launch it. Hence they do not need to immediately react to you having the capability. So I think research into this area, preparations, and development of the system are fine. Heck it is even important and desirable. We just need to continuously refuse to even ready such systems outside of well-announced and limited tests.
Indeed, that's the classical game theoretic view. However, consider two possibilities:
- what if deployment of an effective BMD is covert and piecemeal, and is presented as fait accompli? The first to do that wins, and we only have one real contender. You might say that's a very dangerous gamble, but…
- what response do you envision if the cover is blown? Yes, classic game theory suggests launching everything, but let's think about, say, Putin's options. If he launches, he is guaranteed to die very soon and have his country obliterated. If he doesn't, his nuclear deterrent is indeed worth less now, but it's not given he'd ever need it. Both because it might not be needed during his long and happy life, and because the US is not very keen on attacking random dictatorships despite some historical precedents. So the choice for him is between immediate death and potential problems in the future, and hey maybe it's not actually BMD or maybe it's not very effective, right? Covert/deniable deployment helps here, a lot
So between those two possibilities, I think it's not given that an effective BMD would be as dangerous as you suggest.
Piecemeal deployment might work, if not discovered. But the discovery risk is rather large.
When an opponent discovers the deployment, either after-the-fact, or during deployment. They have many responses beyond immediately launching everything. The main problem is that this deployment corners them. Perhaps they try to attack the missile shield in space. Perhaps they run covert operations to disable the shield. Perhaps they take economic warfare counter-measures. I imagine plenty of former allies might side against whomever made such a huge uni-lateral move.
In any case, as we say in my native language, a cornered cat makes weird jumps. I don't think it makes sense to corner China or Russia in this field. MAD works well enough.
Even if they're defensive weapons only? I think Reagan's star wars program was cancelled only because the Russian threat was not as big anymore. Not because of weapons in space concerns.
Democrats / Clinton shut down the original Star Wars program because they favored arms control (arguing Star Wars was illegal under the Anti Ballistic Missile Treaty)
That treaty expired in 2002 and then Elon went to Russia to buy some ICBMs...
They also shut down star wars because it relied on magic that didn't exist. Raegan was a big fan of big projects that didn't make technical sense (see also the SSC)
Weapons are bad in general. The military, however, takes a practical view rather than a moral one, and will not limit themselves based on what is good and bad, but will arm themselves based on what is effective. This is why we can't have nice things.
They may be bad in some game theory way, I don't know the argument, but large groups of humans are very much not rational actors. The tendency is to oppose the enemy rather than to maximize gains or minimize losses.
