So I conclude that unless something very important happens with China's leadership or cultural/social practices, the economy of China will collapse by 2035 and China will then disintegrate into a civil war. I'm going to put about a 80% probability estimate on this one.

The "unless something very important" hedge makes this prediction rather hard to judge: it's vague enough that anyone might confidently predict "something very important happens with China's leadership or cultural/social practices between now and 2035", because something is bound to happen that arguably falls under this specification.

This is kind of a converse rule of thumb to the "prefer status quo", which - I should maybe have said - is valid at fairly short timeframes. Over long enough timeframes, the reverse is true - pretty much everything we can specify is going to change. (Over long enough timeframes, continents will move, mountains will be eroded to sea level, and eventually the planet itself will no longer exist.)

Over a twenty year timeframe, if you phrase your prediction in terms of (say) "the rate of exchange between USD and CNY", there is some chance that one of the currencies will cease to exist, or that the notion of "rate of exchange" will stop making sense.