Impact energy grows with the square of the velocity. If the velocity doubles, that is like quadrupling the mass of the bullet, which is like turning 20mm ammo into 32.75mm ammo.

It's not like a small flightless ant on radar, and anyway a Mach 4 object will be hot from compression heating. Mach 4 is just below what it takes to rip air molecules apart. That's damn hot. IR tracking would be trivial.

There is no limit that says "one bullet only". Typically the Phalanx CIWS fires about 100 shells, and those aren't even equipped with seekers. It's just dumb 20mm shells, about the same as carried by the typical fighter plane, costing about $30 each. We use the Phalanx CIWS against incoming missiles. It works great.

BTW, that was an unimpressive video with no clear point. Yes, missiles can fly and go after targets. We all know this. It works especially well in the missile vendor's promotional video.

I think you're forgetting that a lot of anti nuclear defense systems use nuclear weapons themselves. When you only need to be within a few hundred meters of a target accuracy suddenly doesn't matter at all, especially when you have several hours to predict the flight trajectory of the enemy missile.

If laser based CIWS turns out to be viable on aircraft then we would see a focus on bigger aircraft that can defend themselves against missiles but even then they would not directly fly into a missile if they can avoid it.

The systems to use nuclear warheads against incoming nuclear warheads were decommissioned half a century ago. I don't think that was ever in use against satellites.

Lasers are nice, but definitely not needed. Phalanx CIWS uses about 100 unguided bullets to destroy an incoming missile. Modern US fighter aircraft carry the same diameter ammunition. The plane is like a flying CIWS, minus the software.

Eh, maybe minus the software. If the software existed, it might not be public. Maybe the F-22 or F-35 already have the capability, limited mainly by pilot G-force survival.