There are plausible reasons to decrease populations in regions or entire countries. With climate change getting more serious, we'll soon start having to move populations away from flood prone areas or areas with increasing desertification. Many of those towns and cities started their own infrastructure projects paid with bonds that assume a certain population, and thus tax base, growth.
Younger generations, for several reasons from despair about ecological future to high costs of living to stagnant wages, are opting to not have children. Other countries, like Japan, have struggled economically because of declining birth rates.
If we (the US) had a more open borders policy with an easier path to citizenship, we could use immigrants to boulster our population and tax base.
With migrating and shrinking populations, I can't think of other economic instruments to pay for local infrastructure projects that have routinely been paid with long-term municipal bonds.