Something else to consider, Our location in the world.
We have no airborne threats to the east, south or west... Everything we have to worry about is north of us. Of all those the most capable air force is allied with us along with a less capable one also an ally. Every other one is too small, under supplied or fielding very old equipment.
We have no need to field more then 100 modern combat aircraft, anything more would be over kill and taking away valuable resources from other area's that all vitally need those resources.
When you break it down those in range to attack us (only really ASEAN members) ..
Brunei - No combat aircraft
Cambodia - No combat aircraft
Indonesia - Only capable combat aircraft are 25 older model F-16's and 16 Su 27/30's, Simply outnumbered by us
Laos - No combat aircraft
Malaysia - Arguably more capable then Indonesia with 18 Su-30's and plan's under way to replace 30 Mig 29's and F-5's with either Typhoons and Rafales, Also a member of FPDA with Australia so more likely an ally then an enemy.
Myanmar - Has a number of modern aircraft including 31 Mig 29's and 16 JF-17's on order however capability is in question due to it's lack of any use in a number of skirmishes (ie: Flashy equipment, no support and experience to use it).
Philippines - While nothing official is more of an ally, or at least friendly to Australia. That said there most capable aircraft are 12 FA-50 trainers from South Korea so of no threat what so ever.
Thailand - Decent force but has tended to be more COIN orientated, 51 F-16's, 30 F-5's and 7 JAS 39's.. F-5's and F-16's are outdated, JAS 39 are there most modern aircraft but too few in number.
Vietnam - 11 Su-27's and 35 Su-30's make up there biggest capability, but between the range, limited budget support and the fact they hate China more then Australia likelihood of them becoming an active enemy is low.
All in those with the capability aren't on bad term's with us being either neutral or close allies, every nation that could become a threat is either too far away, has no airforce to speak of or too little of one to matter.