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There will of course be some economic hurdles to over come as constant and unsustainable growth is done away withFor a long time, I've been hoping to see better theories of "managed decline". (Or perhaps "managed contraction", to avoid nastier implications.)
There's room to bicker about what economics says will happen to per-capita income as population rises and falls - does lost comparative advantage outweigh decreased marginal costs of goods? But it's pretty obvious that none of this theoretical stuff is sufficient to drive what we actually see, where even modest shrinkage leads to economic and often social collapse.
The history of the American Rust Belt is a clear demonstration of why we need better approaches to infrastructure and administration in the absence of population growth. Depopulation there has been a scattershot process that creates empty houses and half-empty neighborhoods, so the tax base has fallen far faster than the cost of providing services like garbage collection and street repair. What's more, the emptying houses are often foreclosed or simply abandoned, leaving them the responsibility of the city. They go unmaintained, driving down property values, and eventually decay until they're uninhabitable liabilities. In places like Detroit, the abandonment rate grew so high that the city couldn't even afford to tear down unsafe properties, creating attractive nuisances and environmental problems. Worse still, many of those cities had debt-financed expansion based on projected population growth, so they immediately and inescapably went broke when it stopped. Very broadly, shrinking to a given population is far more expensive and damaging than rising to the same population.
That doesn't mean shrinkage is inherently bad, I don't think. But we're still terrible at accommodating it, and I haven't seen many good plans for how to do better. Most cities in the US are functionally insolvent with flat population and modest tax hikes, much less shrinking population. For a lot of first-world countries, putting together viable financial and logistical approaches to shrinking populations is going to be a key challenge for the next 50 years.