But I didn't bite any of the counterarguments to the extent that it would be necessary to counter the 10^100.

I don't think this is very hard if you actually look at examples of long-term investment. Background: http://www.gwern.net/The%20Narrowing%20Circle#ancestors and especially http://www.gwern.net/The%20Narrowing%20Circle#islamic-waqfs

First things:

Businesses and organizations suffer extremely high mortality rates; one estimate puts it at 99% chance of mortality per century. (This ignores existential risks and lucky aversions like nuclear warfare, and so is an underestimate of the true risks.) So to survive, any perpetuity has a risk of 0.01^120 = 1.000000000000001e-240. That's a good chunk of the reason to not bother with long-term trusts right there! We can confirm this empirically by observing that there were what must have been many scores of thousands of waqfs in the Islamic world - perpetual charities - and very few survive or saw their endowments grow. (I have pointed Hanson at waqfs repeatedly, but he has yet to blog on that topic.) Similarly, we can observe that despite the countless temples, hospitals, homes, and institutions with endowments in the Greco-Roman world just 1900 years ago or so - less than a sixth of the time period in question - we know of zero surviving institutions, all of them having fallen into decay/disuse/Christian-Muslim expropriation/vicissitudes of time. The many Buddhist institutions of India suffered a similar fate, between a resurgent Hinduism and Muslim encroachment. We can also point out that many estimates ignore a meaningful failure mode: endowments or nonprofits going off-course and doing things the founder did not mean them to do - the American university case comes to mind, as does the British university case I cite in my essay, and there is a long vein (some of it summarized in Cowen's Good and Plenty) of conservative criticism of American nonprofits like the Ford Foundation pointing out the 'liberal capture' of originally conservative institutions, which obviously defeats the original point.

(BTW, if you read the waqf link you'd see that excessive iron-clad rigidity in an organization's goal can be almost as bad, as the goals become outdated or irrelevant or harmful. So if the charter is loose, the organization is easily and quickly hijacked by changing ideologies or principal-agent problems like the iron law of oligarchy; but if the charter is rigid, the organization may remain on-target while becoming useless. It's hard to design a utility function for a potentially powerful optimization process. Hm.... why does that sentence sound so familiar... It's almost as if we needed a theory of Friendly Artificial General Organizations...)

Survivorship bias as a major factor in overestimating risk-free return overtime is well-known, and a new result came out recently, actually. We can observe many reasons for survivorship bias in estimates of nonprofit and corporate survival in the 20th century (see previously) and also in financial returns: Czarist Russia, the Weimar and Nazi Germanies, Imperial Japan, all countries in the Warsaw Pact or otherwise communist such as Cuba/North Korea/Vietnam, Zimbabwe... While I have seen very few invocations recently of the old chestnut that 'stock markets deliver 7% return on a long-term basis' (perhaps that conventional wisdom has been killed), the survivorship work suggests that for just the 20th century we might expect more like 2%.

The risk per year is related to the size of the endowment/investment; as has already been point out, there is fierce legal opposition to any sort of perpetuity, and at least two cases of perpetuities being wasted or stolen legally. Historically, fortunes which grow too big attract predators, become institutionally dysfunctional and corrupt, and fall prey to rare risks. Example: the non-profit known as the Catholic Church owned something like a quarter of all of England before it was expropriated precisely because it had so effectively gained wealth and invested it (property rights in England otherwise having been remarkably secure over the past millennium). The Buddhist monasteries in China and Japan had issues with growing so large and powerful that they became major political and military players, leading to extirpation by other actors such as Oda Nobunaga. Any perpetuity which becomes equivalent to a large or small country will suffer the same mortality rates.

And then there's opportunity cost. We have good reason to expect the upcoming centuries to be unusually risky compared to the past: even if you completely ignore new technological issues like nanotech or AI or global warming or biowarfare, we still suffer under a novel existential threat of thermonuclear warfare. This threat did not exist at any point before 1945, and systematically makes the future riskier than the past. Investing in a perpetuity, itself investing in ordinary commercial transactions, does little to help except possibly some generic economic externalities of increased growth (and no doubt there are economists who, pointing to current ultra-low interest rates and sluggish growth and 'too much cash chasing safe investments', would deprecate even this).

Compounding-wise, there are other forms of investment: investment into scientific knowledge, into more effective charity (surely saving peoples' lives can have compounding effects into the distant future?), and so on.

So to recap:

1. organizational mortality is extremely high
2. financial mortality is likewise extremely high; and both organizational & financial mortality are relevant
3. all estimates of risk are systematically biased downwards, estimates indicating that one of these biases is very large
4. risks for organizations or finances increases with size
5. opportunity cost is completely ignored

Any of these except perhaps #3 could be sufficient to defeat perpetuities, and I think that combined, the case for perpetuities is completely non-existent.