- rising cost of living, propelled by foreign investment in housing, pricing out even upper middle class
- more taxes
- more crime, and less policing
- shitty schools
As soon as you don't need to be in the expensive city for work, it's like hmmm, why the hell am I putting up with this? People with kids may be slightly less flexible, but anybody young and single can basically move very easily. People forget that companies can move easily too.
In terms of amenities, city governments forget that the whole microbrewery/dispensary/expensive coffee shops/hot yoga/avocado toast ecosystem has basically been franchised out at this point and is trivial to stand up anywhere. Sure, it's not an opera house in Manaus, but lots of cheap places in the US have more culture than Bellevue or San Jose.
Expensive cities will argue people will return, and sure, some will, unless the city is too dangerous and completely broke from the tax base leaving. If you are young and single, and want to be around other young single people, and let's say a little midwest spot becomes hip and has the basics, and lots of people move there, you don't really need NYC (at least not everybody). And $500/month rent for a nice 1 bedroom (or a mortgage) is appealing.