The thing you have to remember is that modern jet fighters take decades to design, test, and deploy, and they have lifespans measured in decades.
Look at the F-16 - the original spec calling for a fighter with those characteristics was drawn up in 1965. That’s 45 years ago. Production of the F-16 was approved in 1976. The F-16 is still a front-line fighter 34 years later.
Other aircraft still active go back even earlier. There are aircraft active in the U.S. military that date back to the 1950’s.
Not only that, but the trend is growing longer as aircraft development becomes more complex and more expensive. The original spec for the F-22 was drawn up in 1981 - 30 years ago. Production was ordered in 1991, but the first production model wasn’t delivered to the air force until 2003. It will probably be a front-line fighter for the next 30-40 years, and maybe even longer. Hell, the aircraft could conceivably be in service on the 100th anniversary of its original spec.
You don’t plan a modern military based on the threats you see today - you plan it based on threats you might face in the future. We have no idea what the world will look like in 40 years. In 1910, the world looked fairly peaceful. The U.S. military was small, and the major threats were colonial skirmishes, piracy, and the like. Within 40 years, there had been two world wars, and the Korean war was just starting. Empires rose and fell - the Ottomans, the Soviets, the Red Chinese, the third Reich.
And if you think this level of change was unique, just have a look at what happened since the F-22 was ordered - the Soviet Union collapsed, and we celebrated a ‘peace dividend’. By 2000, there was talk of permanent peace and there were no real signs of trouble anywhere, other than the kinds of local skirmishes, piracy, and terror attacks that were the order of the day in 1900. But within three years, the World Trade Center had been attacked, and the U.S. was in two wars in the Middle East.
Looking out at the horizon today, the world looks a lot more dangerous than it did in 1910. China is growing rapidly, Russia is attempting to restore itself to the heights of the old Soviet Union and becoming aggressive in Eastern Europe, Europe itself is being destabilized by high levels of immigration, low birth rates and high debt. Japan is undergoing a demographic transition that could cause it to lose half its population and destroy its economy. China is growing like mad, but there are signs that its growth is unsustainable and could be yet another bubble economy, waiting to burst.
If you remember the conditions that led to WWII, it was similar. Periods of economic turmoil and the collapse of old political and economic orders are destabilizing.
Anyone who looks around now and concludes that the military isn’t really needed, or that it can be a fraction of its current size, needs to go learn some history, and then take a fresh look at how the world really is and what it could possibly look like in 40 years. It could be a radically different place, with threats that are completely unanticipated today.
Unless you’re in a war, it will always look like you have more military than you need, because your military has to be ready to fight the next war. Scaling it way back because you don’t see current threats is the surest way to trigger a war in the future.