Actually, it was solved 100 years ago or so with technology. For example, steel framed buildings and Otis elevators made tall buildings possible and a more eco

Actually, it was solved 100 years ago or so with technology. For example, steel framed buildings and Otis elevators made tall buildings possible and a more economical use of land (eg, less expensive apartments and offices than before these technologies existed). If you lived in NYC, you wouldn't see Airbnb as a solution but rather a problem created. It allows transients into buildings and we don't want that. We value our safety and pay for it financially. The real solution is to undo the laws that create the artificial "economic rents" that benefit landlords at the expense of renters. See Economist Glaeser's column I cited above.