Massive investment and ever increasing market value of abodes is generally the opposite problem of 'urban collapse' - I mean, unless the bridges are falling down, which I doubt.

NYC in the late 1960's and 1970's underwent a kind of urban collapse.

Detroit underwent urban collapse and never came back.

I think this is maybe what the author meant by 'we don't know what this means'.

If companies, middle class and power flee a city, there is a 100% chance of urban collapse due to the lost tax base.

A thriving city like NYC or SF that are a dysfunctional mishmash of 'barely effective' - well that's another kind of problem but it's not quite urban collapse.