Sure, space will be important in the future. I'll give you that.
So there comes a time where space dominance is the most important trait of a superpower.

But what is their rationale for starting a war with China? You can't just say 'they're getting too big for their britches, let's fight them and lose millions of lives to stop them.'
They could start one very easily right now, simply by declaring that they have proof that China released covid as an act of biowarfare.

No, I think it will just be a gradual thing, as China slowly overtakes them. Might take a while, but it will probably happen naturally, without war.
Unfortunately, history has shown that it takes conflict for the balance of power to shift so drastically. It's not just about economics. If it was, I'd agree with you. China can certainly become the next economic superpower, without a war. But are they the leading military power? They probably will become so, if they have sustained economic dominance, but that will take a long time.

Don't think so. It certainly helped, but they hardly had a bigger air force than the UK at the end of WWII.
No, but it was able to, and willing to, deploy nuclear weapons from the air. That was a very dominant act.

Really effective would be a genetically-coded microbe that only kills the enemy race.
That's pretty brutal. We only want to kill the leaders. Better than rods from god would be an army of nanobots.

Who?
Gaddafi.

Well it's an element so you're probably right. If you mean it can't be distilled from urine, I have to admit I haven't looked into it.
Nor have I, I'm basing that off memory and can't be arsed to fact check it.

Not sure destroying a country and killing its entire population counts as a win
Haha says the man who wants a microbe that kills a race! But yes, wiping a nation off the map counts as winning a war. The prize is everyone thinks you're a cunt but won't dare say it to your face.