This series from Strong Towns does a good job of explaining the idea (with case studies). The gist is that the specific pattern of infrastructure design we've been using since WWII (car-dependent sprawl) isn't financially sustainable without relying on future growth. We got away with it for a while due to the rapid growth seen in the 2nd half of the 20th century, but as growth has slowed, we're finding ourselves falling behind.

https://www.strongtowns.org/journal/2020/8/28/the-growth-pon...