Holy thread-drift Batman!
To answer the OP's question, you need to move the POD back to 1930 and seriously fund Whittle's early jet engine experiments. Have that Hungarian (turboprop) designer flee to Britain or Canada during the war. Turboprops would be a quick upgrade to existing piston-pounding airplanes, allowing them to fly higher and heavier, but not fast enough to create compressibility problems. Turbines also reduce the need for high-octane fuel.
Secondly, have European countries retain most of their colonies. This requires a difference Bretton Woods Conference.
Thirdly, have the USA scrap thousands more airplanes at the end of WW2. That would force serious development of airliners - in several countries - immediately post-war. More of those "DC-3 replacement" programs (1950s and 1960s) would enter production.
No Operation Paperclip, would see far fewer German engineers emigrating to the USA. With that talent more widely spread, many more European nations would be able to compete with large American manufacturers.
Also have the USSR suffer worse casualties during WW2, forcing them to focus on rebuilding the USSR until the 1960s. Rebuilding would distract the USSR from all those petty proxy wars.
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As a taxpayer I don't want to have to replace or drastically upgrade the RAAF F35 fleet at a cost of billions in 10 years because we were too stupid and short sighted to spend a few hundred million extra in the development phase.[/QUOTE]"
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Sorry to burst your bubble mate, but mid-life refits are the norm for most weapons systems.
First, new airplanes only have a small advantage when they are delivered. Because potential enemies have been developing new counter-measures as soon as they heard about early stage design of the new airplane.
New counter-measures and new enemies constantly move the goal posts, forcing airforces to frequently respond by modifying airplanes. The RCAF has spent untold millions of dollars updating software and avionics in CF-18s.
A third motivation is wear and tear. Military flying is tough on military airplanes and they crack far sooner than airliners. For example, the RCAF may be flying the same CF-18As they bought 35 years ago, but fuselage barrels have been replaced along with most of the electronics plus all the usual high-wear components like tires and brakes and engines. A CF-18 pilot -trained during the 1980s - would have a tough time flying the RCAF's latest upgrade.
As to why the USMC needs a its own dedicated fleet of ground-attack airplanes ..... Air Force or Navy pilots can never truly understand what misery forced Marines to call in air strikes. Only time-the-mud teaches USMC pilots - turned FACs - WHY ground attack is important. Short lines of communication are vital.
The other reason for VSTOL interceptors is redundancy. If bad guys hack carrier computers, Naval interceptors are grounded, forcing USMC to fall back on their own planes.
Finally, consider export sales. Most third world airforces can barely afford one model of fighter, so that model has to be multi-role. See the RCAF for details.