If the downtown gets abandoned by offices, it can be populated by something else epxloiting lower prices, as has happened countless times throughout the history. Take London's Soho for example: first populated by wealthy families and Huguenots, then abandoned by them and taken over by prostitutes and such, which ultimately made it the centre of entertainment with new theatres and music venues, made cool by gays, and then gentrified again by the wealthy. It's a circle, not a line.