My assumption would be that the attacker builds missiles based on the defenses they want to defeat. If you have no defenses, maybe the defender builds 1,000 missiles. If you have 1,000 interceptors with 100% accuracy, then maybe the defender builds 2,000 missiles.

This is why the superpowers mostly scrapped their ICBM defenses in the 70s. The technology worked fine. It's totally doable with 1970s technology if you're willing to put nuclear warheads on the interceptors. But for every ICBM interceptor you built, the other side could build another ICBM for the same cost or less. And you need more than one interceptor per ICBM since they can fail and the each interceptor only covers a small area. Add in multiple warheads on a single missile and decoys and suddenly you might need 10x or more. So the USA gave up on the idea of covering the entire country with interceptors, deployed a few interceptors to protect some missile silos, then shut it down after less than a year. The USSR built out a system to protect Moscow and only Moscow, which is still operational today. However, the British were able to maintain the ability to defeat that system and destroy Moscow with a single submarine, all on their own, never mind what the USA would throw at it.

If you have a certain amount of stuff you can build and you're deciding what to do with that capacity, it's not at all clear that missile interceptors are a good use of that capacity even if you're protecting objects that cost orders of magnitude more than the interceptors cost. It works if you're defending against a far less capable adversary (Israel's Iron Dome against Hamas, USA's GBI system against North Korea) but not with an enemy that's even vaguely close to being a peer.

That worked in 1970 because there were exactly two players who had incentive to not spend all the money so they agreed to reduce the total ICBMs instead. In the current world there are too many actors - it won't work, they can make thousands of missles. Ukraine has already proven you don't get to control when you are attacked. Thus the only option today is cost reduce defense and produce enough to intercept several thousand per day.

>However, the British were able to maintain the ability to defeat that system and destroy Moscow with a single submarine, all on their own

What are you referring to here?