There lay another complication. You aren’t just training people to fly - that’s a bit like comparing private drivers, F1 drivers, rally drivers, bus drivers and HGV drivers as just ‘drivers’. For military aircrew you have to train them to fly, to fight and to win in really complicated and disparate disciplines and with very different aircraft types - compared to Boeing’s and Airbus’s latest offerings for a very limited set of mission sets (freight hauling or passenger hauling). Layered on top of that you need them to become your commanders and so you have to train them to be Sqn Ldrs and above, or FS and MAcr, therefore they need to acquire management skills and for the Sqn Ldrs and above some ‘staff skills’ too.
As for Pilots wanting to go to BA or Virgin, that dynamic has also changed. The wise folks go into defence, which is where the serious money is, or as someone else already eluded, into some of the various contracts supporting defence with their aircrew (Draken, Ascent, Inspire, BAe, etc…). The terms and conditions of airline flying is certainly less attractive these days! I certainly would not consider going near the airlines, but I did in the late ‘90s when the terms and conditions were way more favourable.
Further, if you go back to 1918 then Trenchard insisted that all officers train as aircrew regardless of what job they go on to do. That was a wise move as not only would many only do a year or so of flying training, and then rarely touch a throttle or stick, they would learn the basics of the RAF’s business and more importantly the Service would have a large bucket of partially trained Pilots to pull from. Fast forward 100+ years and less than 10% have any experience of actual flying across the Regular and Reserve cadre. So you can see how trimming down the number that gain that valuable 2-3 years of flying training has also painted an air force of 31,750 into a corner.
Finally, Reserve Aircrew are fine but it takes money and people to do it; and lots of it if you want to replicate ANG levels of effort. The thing is, if you have to create a set of Reserve flying Sqns then you need to develop the same amount of effort and pay the same amount of time as developing a Regular sqn. That was the conclusion that was rightly realised in the 1950s when the flying RAuxAF Sqns were stood down. If you embed the Reserves on FL units then you create further management problems against taught flying programmes where the Reserve has to take priority over the Regular as they are only turning up that particular day of the week - meaning that the Regular gets booted off of the flying programme when the jet situation crumps. So really Reserve Aircrew can only make up a very small proportion of your effort otherwise they become significant disrupters and difficult to keep safe and competent.
PS. I could probably teach someone to “fly” a Typhoon safely in a couple of months. If I want to teach them to “fight” the Typhoon, effectively as part of a large multi-national COMAO, then it will take me the best part of 5 years! Even to fight as a singleton it will take me at least 3 years in all likelihood if you want them to be victorious over their adversary.