I wonder how much of that (the staying in your big house once empty nest) is a cultural shift. Used to be, once my kids are 18+, in college or college educated, they go out and get a job and have an apt or house with roommates, go live a life. It's not much different than the spartan life you lived as a broke colelge student, but it's a little better and it's a life. Earn some money, gain some skills, go out, go on dates, etc. but be 100% independent. The only time your kids moved back in with you was if something really bad happened and it was kinda shameful. That or there are some fundamental issues like with disabilities or handicaps that precluded the ease of a normal life.
Now it seems like a cultural norm to have your kids stay with you longer, the coddling mother saying "just live here and save some money", while you go to school or work your first couple of entry jobs to pay your dues. It's not uncommon for able-bodied adults to live at home until their 30's these days, which kinda blows my mind, but I get that it's harder than ever for kids to live without roommates in desirable cities and locations so it's not longer the burn outs and losers who live at home, but good kids who need more time and help.
And I wonder how many parents don't sell their big house because they are taking a "wait and see" approach to see if their kid will be able to muster it or if they need the extra rooms for the boomerang in case they need some more time and help.