There's a reason many of the Gilded Age estates are now public properties, they are massively expensive to maintain and property taxes are likely insane. It was one thing for the Robber Barons to pass them to their children as they were likely to be wealthy, but I just don't see it for these $100M houses. I can see them maybe being owned by insanely rich foreigners looking to park money, but then again to people like the developer I'm probably just a poor guy that lives in a house that is only 1600 sf.