You can't bootstrap having a nice walking environment at above-ground level that way. Who's going to open the first cafe where there's no foot traffic? Why will anyone spend a lot of money on entrances that no-one's going to walk to? What pedestrian is going to want to keep walking up and down stairs while the network is partially complete. The City of London tried to do what you're suggesting and it was a total failure, because while it might have worked if it could have magically all been done in one go, there's no way to get to there from here.
