First, there's this:

What's defensive is this: that we have over 2,000 nuclear-tipped ICBMs, ready to fly at a moment's notice.

No, we don't have 2,000 nuclear-tipped ICBMs. The actual number is more like 500. We also have about 200-300 Trident SLBMs. That's still a lot of firepower but not even half the 2,000 you claimed.

Missiles come in many sizes. Our launch detection satellites can pinpoint a launch site very soon after ignition, so it isn't possible for someone to launch without us knowing about it. However, they don't have to launch from their own country or even have an ICBM capability. Shortly after WWII, the US Navy proved you could launch V-2 rockets from the deck of a ship. A rocket with a range of a couple hundred miles (like a Scud) and armed with a nuclear warhead could be launched from any number of ships, including container ships. That's just one possible scenario of how even well financed terrorists could attack the US with missiles. Developing nuclear armed missiles isn't the sole domain of superpowers anymore.

Like I've written before, a nuclear missile capability in the hands of someone like North Korea can be used for nuclear blackmail. You didn't consider the flip-side of your scenario about the US not invading North Korea because they could nuke Seattle. The flip side is what would happen if they choose to invade South Korea and threatened to attack Seattle if we opposed them? The same applies to China invading Taiwan. China knows we aren't going to invade them. The logistics of invading China make what's going on in Iraq look like child's play. For the same reason, they can't invade us (and neither could the Soviet Union even when it was at its peak of military power).

China currently only has a dozen or so ICBMs capable of hitting the US. North Korea's ability is even more limited. A layered "limited missile defense" system can be built that would offer a high probability of stopping all of those missiles. That's what they're upset about - the blackmail capability they've spent so much money developing could be negated before it could be used.