The premises in this argument aren't strong enough to support conclusions like that.

Sure. But the support for other parts of the perpetuity argument like long-term real returns aren't strong either. And a better model would take into account diseconomies of scale. Improbability needs to work both ways, or else you're just setting up Pascalian wagers...

Expropriation risks have declined strikingly, particularly in advanced societies,

They have?

and it's easy enough to describe scenarios in which the annual risk of expropriation falls to extremely low levels

Even easier to describe scenarios in which the risk spikes. How's the Middle East doing lately? Are the various nuclear powers like Russia and North Korea still on friendly terms with everyone, and nuclear war utterly unthinkable?

ETA: Weitzman on uncertainty about discount/expropriation rates.

This seems to be purely theoretical modeling which does not address my many disjunctive & empirical arguments above against the perpetuity strategy.