It becomes a big problem when City policies create living conditions that are so bad that anyone with the resources to do so, leaves the city.
Detroit was the canary in the coal mine. Detroit if memory serves is at 40% of its peak population. Detroit has been deconstructing itself in an effort to restore livability to the city.
We are experiencing the death of the Republic.
COVID hastened a situation that was looming for most cities: non-manufacturing jobs, due to technology, no longer needed to be situated near ports (whether on the Atlantic/Pacific coasts or the Great Lakes). On top of that, they have a massive ageing infrastructure (including their expensive government employee caste), accompanied by huge spending on people no longer sending tax revenues into the city coffers (as the flight of jobs accompanied the flight of property taxpayers and city payroll taxpayers).
The push for “national socialism” (sound familiar?) is driven by the realization that as long as companies and workers can flee to greener pastures in “free states”, then the solid socialist areas are doomed - the geese that laid the golden eggs have fled, leaving just the outstretched palms of the permanent underclass and the government worker caste “administering” them.