I think there is a bit of a 'false choice' where a decision on retaliation has to be made NOW when it was only a single missile, a response can definitely wait for a single nuclear strike that does not hold the entire at risk.
That is an extremely complex targeting problem, much more so when the Sprint was in service in the 70's. That is why GBI's intercept the missiles much earlier in flight, better chance to hit less targets. You can at least partially solve the targeting problem by arming the interceptors with nukes themselves like the Sprint was but that causes a whole host of additional issues, and and even then it still doesn't solve the problem if the missile is MIRV'd and/or has countermeasures.
Missile defense is a hard problem to solve, that is the biggest reason it costs so much money.