I'm not sure why you say this. Germany and Italy didn't collapse-- they were conquered. The Soviet Union did collapse, it's true, but it collapsed because of external pressures and sclerotic bureaucracy causing an inability to adjust to internal problems. In all three cases there was an external force contributing to the problem. However, there's no reason to assume that China's authoritarian rule will collapse as well. Indeed, in several Soviet successor states (including Russia), authoritarianism has reasserted itself.

Authoritarianism is easy to impose and democracy is fragile; the civic discourse required for a strong and stable democracy relies upon a considerable amount of mutual respect and rhetorical restraint.

South Korea had decades of dictatorship with a relatively easy transition to democracy and liberal capitalism.

That's true. It's the maintenance of that democracy that's hard, though. Plus, it was by no means certain that the country would become a democracy.

External pressure as one of reasons of USSR collapse? Come on, really. You can't be serious. It had lived under much more strong pressure more than 70 years before authority decided to demolish the union. What pressure do we speak about? Commodity prices decrease? I can't believe the west is involved in the free market manipulations though. Unthinkable.

> The Constitution was designed to make the US Government as weak as possible, because a strong government ultimately turns on its people.

This is false, though it's a popular modern myth; the whole reason for the Constitutional Convention was to strengthen the government under the Articles of Confederation to address problems caused by it's weakness, and the framers decided they needed to go a lot further than minor tweaks in doing that, sure some of them wanted to go even farther—Hamilton proposed an elective monarchy, for instance—but even the more modest final outcome was neither in intent or fact “as weak as possible.”

These are consistent, under a particular reading of "as possible".

The US has been an instrumental part of a system where we all benefit when others are successful, it is not a zero-sum game. China's economic success is good for America. What is not good for an America is a new xenophobic super power that doesn't share our values.

A stable, prosperous, democratic China is ideal. An easy way for an authoritarian government to keep internal dissent down is to make an external enemy. Think of all the examples you mentioned. So this is probably bad news for the U.S. and the whole world.

> authoritarianism has never worked.

Neither has democracy, depending on the definition of "worked". Try to narrow it down.

> The Constitution was designed to make the US Government as weak as possible

That is a premise, not a truism. Every democracy has, in time, given in to an appeal by the masses to (either by dupe or passively) give the government more power until it's not relatively weak.

I wouldn't be so quick to write them off. A large percentage of the world respects the Chinese government which usually acts in their own economic interest is stable over the long term. Whereas in the US the government does nothing but enact poor economic and social policies as well as saber rattling or war every time a right wing president is elected.

Well, unless you are Vietnam, the rest of SE Asia, Japan, or Taiwan. China can and does rattle sabers also.

Or India, or South Korea, or the Philipines..

China has quarrels with almost every nation that surrounds it. Even nations too far way to be affected by the sabre rattling are starting to question China and their influence (ie the debate in Australia right now).

I think you're underestimating the strength of the US government. A lot of things happened since the 18th century.