Missile Defense concept

(Without Stag there probably isn't going to be any good discussion in this thread, but here's to trying).
Okay, imagine this: a country monitors the airspace on its borders. High-quality sensors that can see into the atmosphere and various other equipment will be constantly taking data, and every millisecond of this data will be recorded.
Whenever a hostile missile enters their airspace and is detected, the recorded data will tell the military at what exact moment the missile entered their airspace, at what point it entered their airspace, what exact direction it's traveling in, and what its speed is.
All of this data is transmitted to a central supercomputer, which calculates exactly what time and exactly where to aim an anti-aircraft gun which would shoot down the missile. It'd then transmit this data to the anti-aircraft gun which was in the best position to take out the missile, and it'd aim and fire automatically.

Dassault Papillon: How is this different from other missile defense systems?

admin: Missile defense typically involves using a missile to shoot down another missile. This method's effectiveness is highly questionable, and it's enormously expensive.
On the other hand, computer systems are not all that expensive. The world's most powerful supercomputer (Sunway TaihuLight) cost about 273 million dollars. A single THAAD unit can cost somewhere near this. AA-guns are relatively cheap as far as weapon systems go.
Furthermore, electronics are arguably the fastest evolving field of technology as of right now; it's arguable that with the right software the average commercial laptop could calculate a typical nuclear missile's flight path and time-to-impact. Just think what a supercomputer could do.

Dassault Papillon: Wouldn't you need anti-missile guns virtually everywhere a missile could possibly land though? I was under the impression missile defense usually used missiles because they have better range.

admin: Seems like one AA-gun per square mile would suffice.

Dassault Papillon: Even that's a LOT when you're talking about a country with many thousands of miles squared.

And the sensors and everything, I think missiles would definitely work out cheaper in 95% of cases.

Dassault Papillon: How realistic is this?

admin: Dang, you're right. Even Israel would require 8500 of them to cover its whole territory if each one protected one square mile of airspace. It could still be used to protect major population centers, though.