To add to what
@ProjectEnd said above; the City's ModernTO program which is seeking to cut back on total office space and make more efficient use of City properties is focused on:
1) Getting City/Agency staff out of leased property and into (City) owned property
2) Consolidating staff into fewer, larger locations so as to save money, increase collaboration and reduce time lost to staff shuffling between different buildings/sites for meetings.
3) Generally (but not universally) shifting staff to downtown or the former Civic Centre spaces in order to offer a more enticing work environment vs more suburban office building offerings.
***
The City is already unloading:
277 Victoria downtown and the current TTC HQ site.
Selling off Metro Hall would be inconsistent w/that vision/strategy.
Though, in the longer term, I could envision a plan to build two larger format towers on the current site, selling one off to the private sector to defray part of the cost of a new 50-storey class-A tower for the City.
But that is definitely not a near-term plan.