We could probably string out 20 billion humans on a diet of 90 of rice and vegetables a day, but why? Shouldn't we be concerned about our collective quality of life?

Alternatively, it's not clear that the current low density of population is actually sufficient to ensure a reasonable quality of life either.

I have this image in my head of a pre-historic hunter-gatherer who's worrying how future people can stand living in cities, with their crazy indoor plumbing and prepackaged food available in giant walk-in refrigerators etc.

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$E(X_t|F_s) = X_s,\quad t > s$