[zanny]

Populations don't decline uniformly and people don't naturally optimize for infrastructure use and the best use of existent resources.

You don't have to even wait for population decline, we already have more unoccupied homes in the US than homeless by an order of magnitude, but all those homes are in "dead" areas where industry or resource extraction came and went. Now those houses and all the infrastructure around them decay without use, while elsewhere infrastructure is overburdened with concentrated people.

If anything, declining population means more per-capita expense on infrastructure, and the per-capita costs of maintaining current infrastructure (at least in the US) is out of control.[1]

[1]: https://www.strongtowns.org/journal/2017/6/4/this-is-why-inf...