Biggest question: how does Fabrice's analysis play out for someone that isn't extremely wealthy?
Yes, if you have $20 M dollars and are talking about simply allocating assets he is probably correct. However, what about for the avg working person who's buying a house with money that isn't his (a mortgage) so that by the time he retires, he isn't burdened with paying for a place to live.
Also, as an aside: I think most people don't have the personal fiscal discipline to take the money they would save by not paying a mortgage and invest it--they spend it. A mortgage is like a forced savings account for many.