Collapse changing stability by its very definition, I do not argue this. But honestly I believe that we moved on in history and current Germany is not looking for blitzkrieg in case when Russia and USA will keep minimal forces.
Its realy nitpicking from your side. Economy capacities and knowledge of nuclear bomb substituting real nuclear bomb but I accept that its useful keep some for better effect. Army should be also useful in case of zombie apocalypse or alien invasion. But the spending is ridiculous.
It's only ridiculous if you can't afford it; and the US most definitely can afford it.
And let's say Russia, US, and China give up their armies (or at least scale them back to the point they have no power projection capability). What then? What guarantees do we have that every tin-pot dictator and warlord out there isn't going to start attacking their neighbors because they know there is no greater force out there to counter their aggression?
Unfortunate as it may be, large military forces from powerful nations are what keeps the peace in this world. A lot of regional conflicts don't happen because the potential belligerents are afraid the US, Russia, or some other powerful military will intervene.