system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: What was the original motivation behind the F-35 program that would have justified how much it's been funded?

Setting aside the infamously ridiculous amount of delays, ballooning costs and hardware problems the program has had, I've been wondering: what possible motivation could the Pentagon have had for sponsoring a program designed to spend tens—if not hundreds of billions—of dollars even by its original estimate? What was wrong with the current fighter jets like the F-22 and USAF fleet that they needed to be replaced with hundreds of these new fighters? Why was this necessary when most of the conflicts the US finds itself in now don't seem to rely much on fighter superiority?

Not being partisan and not looking for partisan answers about government bureaucracy or whatever: I'm just trying to understand the Pentagon, Lockheed and Congress's motivations in commissioning this thing in the first place—and why they thought it was worth sticking with.
gpt
Jets have a limited lifespan; as they fly and pull Gs, heat up and cool down, their materials eventually fatigue, crack and break.

The F-16, Harrier, A-10 and F/A-18C/D are all approaching their use-by dates. The F-16 in particular is the main jet of the USAF, the F/A-18C/D makes up half the Navy / Marine air combat fleet and the Harrier is the only jet that can operate off Marine Corps LHDs / LHAs.

In addition, Russia, China, Iran, North Korea, etc have continued to develop new weaponry (tanks, jets, air defences, etc). Sure, the US is not at war with either, but militaries are designed to fight the wars you can't afford to lose. That means you're just wasting money if you intend to buy more of the same old aircraft.

A big factor in all of this is that it takes a long time to develop a modern fighter - the first US jet fighter took <18 months to go from concept to flying. A modern fighter (F-22, F-35, Typhoon, Rafale, PAK-FA, J-20, etc) takes about 20 years to go from concept to in service. To just manufacture an already-designed fighter takes about 2 or 3 years. 

Combine that with the expectations that a near-pear war (eg NATO vs Russia or China) would be a lot quicker than WW2, and also the fact that the supply chains of these jets reach around the world, and you end up with a situation where if you're going to fight a war, you're only going to be able to fight it with what you already have.

In the case of the Joint Strike Fighter (F-35), the US is building a "5th generation" (same as the F-22) strike fighter that provides qualitative (it's been getting >20:1 kill ratios against F-15s and F-16s) and quantitative (the US + allies are buying >3000 F-35s; that's more than Russia and China's fighters combined) advantage to ensure that the US / NATO can control the skies and win any conventional war.

If you're thinking about nukes at this point, you have to understand that the understanding of the global and military effects of nuclear weapons today is much different to what it was 30 years ago / during the Cold War. The Soviet Union believed that after they and the United States exchanged nuclear missiles and bombs, that they'd be able to have a considerable number of forces survive, with those forces then being sent out across Europe to try and conquer as much as possible. 

Today we're aware how unrealistic that scenario is and so even if WW3 broke out, there's a decent chance that nuclear weapons would be used in a more sparing fashion, if at all (look up NUTS - Nuclear Utilisation Target Selection). If you're Vladimir Putin, etc, you'd probably like to retain the option of having you and your family escape to some secluded island or 3rd world country without worrying about radiation and dramatic climate change destroying your hope of survival.