I really like the comparison with the n-body problem and now even think that the block size could in fact stabilize around an equilibrium point. It seems that there must be a block size (the central mass) where no individual miner has an incentive to deviate by building smaller or larger blocks.
But the point was that is not a stable equilibrium because the profits are not accruing proportional to hashrate due to unequal propagation or orphan rates (i.e. wasting more hashrate). Thus over time the hashrate concentrates until there is a 33 - 50% attack and then eventually the cartel can set any block size it wants with a 50% attack.
That is why Monero's design also will end up centralized.