I think you might be conflating two issues. Housing in downtown isn't desirable for a populace of people (probably true for those who need more space and/or want to think about issues like schooling etc.) and downtown isn't being used (even for office use).
The idea to turn office towers into apartments tries to address the second issue without explicitly trying to solve the first (which is make downtown more desirable to live in).
I'm not conflating them at all. They are two interdependent variables in the system. Even TFA says that 20% usage of the empty buildings would be extremely optimistic. So you're not going to be even close to solving it by focusing on just the second.
The idea to turn office towers into apartments tries to address the second issue without explicitly trying to solve the first (which is make downtown more desirable to live in).