> Infrastructure costs increase as you increase density.

They increase much slower than the efficiency gains from having a higher density and the rate of per capita costs decrease from having much more people pay for those costs.

When a city passes ~20,000 people per square mile the per person cost of infrastructure increases as roads stop being efficient. You start needing tunnels for roads and or subway systems. You also need deeper pipes for water etc, it’s not a deal breaker, but it is a real issue.

Having existing residents subsidize new residents is a common approach, but understandably unpopular.

Where does this 20,000 number come from?

Infrastucture costs are higher for decreased density. This is well known. You have to lay a lot more pavement and pipe when people live farther apart. Your typical suburban subdivision doesn't even pay enough in property taxes to maintain their infrastructure, and is subsidized by the denser areas. The folks at StrongTowns have done tons of work in this area.

If the comparison where to empty fields or low density suburbs I would agree per person, but we are talking about existing cities which act as an inflection point. Further, expanding infrastructure is an additional capital cost where much existing infrastructure has already been built and paid for.