Reward-seeking intelligences might have innate features which prevent exponential expansion though and instead result in expansion-contraction loop. For example

Reward-seeking intelligences might have innate features which prevent exponential expansion though and instead result in expansion-contraction loop. For example, for every new colony, the probability that any single colony will begin a civil war may increase. Or the probability that a supervirus will be manifested somewhere and then spread along the galactic population of that species. Basically, if the species are uniform in some manner, the more colonies they have the larger the probability that some colony will spawn an event which utilizes their uniformity to spread and destroy most of the galactic population of that species. Obviously it's not a guarantee, but I believe it is a viable theory as to why the world described in the article may be possible.