I agree with most of your assessment except this one. When the USSR collapsed, was there any war that followed?
The USSR wasn't acting like a petulant child like the US does, and doesn't believe it is a powerful force for good in the universe so the universe owes it if something does go wrong.
If today Russia or China collapse, will WWIII start? most definitely not.
You seem to have worked this all out already... if Zhirinovsky or some extreme nationalist like him but 30 years younger took over in Russia and started acting way worse than the west accuses Putin of doing now... who actively interferes in western elections like the US actually already does, and murders people openly like Saudi Arabia does, conflict would certainly ramp up and there would be many in the west demanding that there is no room on this planet for two US of As.
What if a similar character takes over in China and turns it into a war economy to build ships and tanks and most importantly nuclear ballistic missiles... tens of thousands of them and then decides to test the waters by invading Taiwan?
Could easily lead to WWIII.
But suddenly when its the US that goes down the entire world burns?
No. In fact I think the world really can't continue with the US trying to be the centre of attention and the worlds policeman and bringer of democracy.
What I think is that if it has its crown taken away and its free lunch no longer exists and it has to again work for its income and actually do something about paying its foreign debt and not with those worthless dollars they print like confette, that a serious tantrum and nuclear weapons use could be a possibility.
I think this is more to do with American fear mongering. I say make it happen then we will see what comes next.
Oh yes... the wonderful... if you don't love it then leave, or even better... if you think this is bad imagine if Russia or China or North Korea were in charge... except that they don't want to be in charge like you so clearly do you pricks.
As long as the US continues to dominate world politics through its economy, it will not stop in its attempts to destroy Russia and China whenever there is an opportunity.
But that is the point... their economy is broken... they broke it years ago... and it needs serious changes that they really don't want to make to get it working again... politicians can talk the talk but when they get into power they owe their positions to the 1% who paid their bills and they certainly don't want to change the system that they became obscenely wealthy from... votes mean nothing when a company can contribute half a million dollars to a campaign fund on both parties sides and then control the eventual president. The system is broken, votes don't matter.
Monsanto can make chemicals that kill birds and bees and children and as long as they bribe the elected officials who decide what can or cannot be used, with direct payments or with offers of cushy jobs at the company with enormous salaries, then those people don't have a chance... that is not democracy.
And it is not like we didn't get warned... in the 1980s Bo and Luke Duke in Dukes of Hazzard, and those lovable rogues from the A Team were always saving pretty women in distress from evil companies wanting to steal their land or poison it or some such... but then companies became good and MacGyver fought evil foreign and domestic government entities to save America... Michael Knight worked for a private company too... the media turned away from saving us from evil corporations to foreign companies and local and foreign governments...