Infrastructure been built out with endless growth in mind.

Unless birth rates rebound, and if population does decline by 4-5 billion over 40-50 years we have more infra than necessary that will just crumble and rot and burn up more resources to maintain

Perhaps you meant saturation of individual needs. That’s harder to understand because such varies and not everyone agrees what are legit specific needs.

But generally we won't need more big infrastructure to account for fewer people actually needing