Accountants have a lot to answer for.
Surely it would make more sense to purchase new airframes at 10-15 years into their service life than it does to spend a fair chunk of the replacement cost on upgrading existing old airframes with limited remaining service life?
Yes. In every major case, the life extension ended up costing more than a replacement, and increased the cost of the replacement. It is not just the ADF, it's a common theme across the West. The Leander MLU is a classic example of something that looked good, but failed miserably in terms of $$ and capability. Now the RN is repeating history with Type 23 replacement - they are built for an 18 year life.... The classic example is for us is the M113 - we have spent the cost of two replacement projects on a fleet that is worse than when we bought it - and that's before you add the cost of LAND 400-3.
Collins is showing this, as is ANZAC. The Hornets came close. Kiowa's avoided the MLU cost, but still should have been replaced in the early 90s. C-130s have avoided this through regular purchases of new, upgraded fleets.
The issue is two-fold - no MLU can match a new build for capability (walk me through the M113AS4's computer aided aiming, its electro-optical sights and it's counter drone capabilities) and the MLU is operating in the wrong half of the bathtub curve. Old things get expensive, and quite frankly, no matter the upgrade, unless you replace everything (which is more expensive than a new build) you still have the old bits there. Also remember, every dollar spent on that old platform is one less for the replacement - Kiowa/Tiger is an excellent example of this where millions were diverted from Tiger to Kiowa at the very start of the program. And that was for a Kiowa, arguably the second most simple platform we have owned (after the bayonet).
20 - 25 years is ideal for a military platform. There will be some exceptions, but pretty much anything expecting to take/give fire is restricted to 20 years. The gap is just too big. People point at the new Apache's as proof we should have got them instead of Tiger - ignoring that the E is two generations beyond the A and, while looking the same, is very different under the skin. Like, 25 years more advanced different. Likewise our new M1s look just like the old ones - expect they are not.
I get the idea of keeping old platforms on the shelf, but these aren't rifles. The require $$ to maintain (our sustainment is already too little for the existing fleet), many $$ to refurb (at the point you will be haemorrhaging $$ in the fight) and you still are facing a modern threat with 2 - 3 generation old materiel. I get the argument - better to have a mediocre F-111 instead of nothing. Except I disagree. If there is a mediocre F-111, it'll get used and we lose two valuable aircrew (who likely have fresh combat experience and in the early days of the war they, like all veterans, are worth more than anything). If there is nothing, the planners will do a different approach - and those aircrew survive to fight back in a new-build F-15F/18J/35D. Reverting to my soap box, would you want to face a PLA-N SAG in Mirage III or Sabre? Because that's what our mech inf are doing. Wouldn't it be better to hold them back, ramp up the production line of AS21, and put them in some new build, modern cars?
Too many people, including my peers, get too sentimental over platforms. 20 years and out. Its what our people deserve, and what our planners need.