The point you miss is that the AFC is rapidly deployable and can react to threats far quicker than deploying ground based units and you are covering only one threat in your argument, An AFC can react to all military threats apart from submarines. An air defence system capable of defending all of NZ would be extremely expensive, as some of the better long range missiles are very expensive for instance depending on the variant a single patriot missile can cost up to $6 MILLION and that is just for a missile, not including the launcher , controlers and radars. You are also assuming the threat in would be slow to build up, but with good planning significantly it could easily larger than our total army before the day was out. The other problem you don't address is the high number of casualties that would be incurred. On top of this what about the sea.
As I have pointed out before , while an economic blockade would be uncomfortable it would be survivable and how are you going to blockade us from Australia? In war desperate measures are taken, governments take total control and losses are accepted, In WW2 3000 merchant sips were sunk by the German Uboats but trade continued.
the reason we get back to invasion is that is how you lose your freedom and sovereignty, that how our way of life is destroyed, It is the worse case scenario and with an AFC is far less likely,. The projected defence idea is great if you have the time when things build up over time, but does have problems if we have a Pearl Harbour pulled on us. As for allies, we currently have only one and they will be tied up dealing with there own problems. As I have pointed out before an AFC does make invasion unpalatable.
On the question of cost, as has been pointed out before, we don't need the latest gen 5 aircraft as in the main they can operate unopposed, just the ability to carry modern anti air and sea missiles. second hand would achieve this a lot cheaper than the numbers of anti air and surface to surface missiles that would be required to give reasonable coverage.
The reasons I like an AFC is that with a modest force you can cover a large area, it is quickly deployable, It is flexible and covers both the sea and air, it puts the least number of lives at risk and it is a very good deterrent which is best as you don't want to fight a war if you don't have to.