It’s my understanding that a lot of cities are built on the bones of previous iterations - even my current major metro, Seattle, is built on top of a lot of old ruins. That doesn’t mean it’s not a problem, it just means we’ve previously been able to conquer it by just… building up from the lower levels. It’s kinda crazy that that’s our best effort for the littoral cities - but I think even Denver is built on some “old” ruins and they were exposed (to my absolute fascination) when they were replacing the streets way down town (like 16th street mall / wynkoop, iirc). I was absolutely fixated on the … legacy city that was going down like 8’ below current denver!
Anyway, I don’t want to act like this fact makes it not a problem; I simply can’t imagine all of lower manhatten wanting to raze every skyscraper and start over either. I’m pretty sure most of these city-on-a-city situations are after major fires kinda force everyone’s hand, so it really is a huge catastrophe!