alt Hacker NewsThere are several different levels of ballistic missiles.
ICBMs, for which the GBI is intended, are the most challenging to defend against and show the least interceptor success.
In contrast, we do have some pretty definitive evidence that theater and "lower" MRBM/IRMB ballistic missiles can be intercepted successfully. If you define "effective defense" as "most missiles that would cause damage are intercepted", then it is clearly possible with current technology. If you define "effective defense" as "all missiles are intercepted", then it remains beyond the current technology.
If you define "effective" in terms of cost ratios: R = (cost of defense system + cost from failed intercepts) / (cost of attack system)
then N < 100 is well beyond current technology, regardless of whether the defense system is perfect or non-existent.
There's no magic Pareto-optimal point where investing the right amount in missile defense means that starting a war against a medium-sized country makes economic sense. Russia figured this out in Ukraine, and the US figured it out in Iran.
Israel's genocide worked pretty well tactically, but is a long-term strategic disaster. If the US continues to be a democracy, polls say that it will cause us to withdraw support sometime this decade. Also, it only works if you have an incredibly asymmetric fight.