Several of my younger friends live in cooperative housing, where they rent a big house, and individually rent out the rooms. Sometimes as many as 8 or 10 people live in a "single family" home. This seems like a great solution for all parties involved (including cities with very high rents), until you learn that the cities they're in often consider this an illegal use of the property and will force their landlord to evict them on short notice if neighbors complain. And, because it's technically an "illegal" living situation, they don't get a lot of the same protections they'd get if they were "legal" tenants. So, leases are kinda meaningless, if the landlord can evict at any time.
It's a sort of housing insecurity that makes their lives a lot more stressful than anyone should have to deal with, and it gives landlords a dangerous amount of leverage over tenants (they charge more, knowing it's being used by multiple tenants, but they'll claim lack of knowledge when the neighbors complain and the city comes knocking). And, of course, they'll ignore the lease if they get a buy offer that they don't want to say no to.
Cities, especially those experiencing dramatic rent increases and that have a history of only allowing single-family homes in much of their livable land (as most US cities do), need to remove zoning impediments to multi-family housing. There was a project published a couple days ago to map the zoning in a bunch of major cities, and it's eye-opening how tiny the available space for multi-family dwellings is in a lot of them, and it makes it make sense that there is a housing crisis in a lot of major cities. Maps are here: https://www.nytimes.com/interactive/2019/06/18/upshot/cities...
Of course, that won't solve the too-big-too-far houses. The other problem with a lot of McMansions is that they were built in the middle of nowhere. Younger people want shorter commutes, the ability to bike or take mass transit, a neighborhood with shopping in walking distance, etc. People have figured out that quality of life in cities is just plain better than suburban life.