This was an interesting read, but it was written mostly as if cities were just surrendered to the forces of nature. In many or most cases, I would assume that when a building starts to succumb to the forces of nature, the city would either reinforce it or demolish it, and the city itself will continue to evolve. In some cases, I supposed cities may be abandoned, but in most cases, it will probably be a slow, evolving change barring some dramatic catastrophe.
Some Detroit skyscrapers sat abandoned for 50-60 years with no heat or electricity. Sure some were falling apart and became a hazard with chunks of brick or concrete hitting the unwary pedestrian but the majority held up fairly well.
When the billionaire Dan Gilbert bought them to rehabilitate he few major structural problems. So I think if major cities are abandoned their skyline may look somewhat familiar centuries from now.
I learned recently that reinforced concrete buildings only last about 50 years. Moisture and oxygen seeps into concrete and corrodes the rebar.
I was expecting it to be about rigid zoning laws or HOAs or something else preventing cities from changing or adapting.
I was big proponent for mass timber until engineer told me the lifespan might be poor due to adhesives in fabrication. Can't find any solid analysis though.
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I always wondered how you go from a thriving city to an 'ancient ruin'. A lot of cities in India have been continuously occupied for over 2000 years (Kalyan, Ujjain, Bharuch, Patna). A lot of them don't have any obvious ruins from that era (most old buildings tend to be from the medieval era or rock cut caves/temples/monasteries that were forgotten about in later eras) compared to a place like Hampi or Angkor (although both cities were abandoned in the medieval era).
My guess is that for a city to have relatively well preserved ruins, it has to be completely abandoned due to some calamity. If a city is continuously occupied, old buildings get incrementally replaced by newer buildings, and very few old buildings get preserved.
Even with Rome, my theory checks out. We have a lot of Roman ruins because Rome was heavily depopulated after the Roman empire collapsed. A lot of buildings were probably abandoned and then buried by dirt over time (like the Roman Forum). The only other preserved ruins were buildings that were repurpose d (Agrippa's Pantheon, Colessium, aqueducts, etc).