The approach and subsequent missed approach are very carefully surveyed and designed to protect the aircraft from terrain provided the aircraft and pilot can meet the performance requirements. In the case of the ILS approach, the performance requirements are to remain within half scale deflection. The missed approach begins at the missed approach point and is based on 2.5% climb gradient as well as tracking and turning splays which are not spelled out in the AIP or on the chart. As I have already pointed out the missed approach point on an ILS is where the applicable DA intersects with the glide path. The applicalble DA in Cairns is 320 feet and that is basically the middle marker. The chart actually specifies the MAP is MM! This is how the approach is designed. If you increase your DA to meet the 2.5% you still need to track to the applicable DA missed approach point to remain within the surveyed approach. This applies to non precision approaches as well. If you turn off the approach early you will no longer be protected from terrain or other traffic.
The Cairns 9 departure heading ranges from 350 to 030 and west bound traffic generally overfly the ILS. If ATC expected that ILS traffic can turn left at any time then they would not clear any departures to the north or west while there was any inbound aircraft.
What would you do if you lost the loc outside 15 miles? The procedure says track 015 and fly along the 15 mile arc. Yiu will be tracking away from it in this case. Extreme I know but I'm simply following some of the 'logic' here.
I would love to hear from a Cairns ATC on this subject.