[quickthrowman]

The value of a commercial building is based on the potential rent, not how much space is leased.

Loans can be called by the lender if the value of the collateral (building) falls too low.

Lowering rents lowers the building value. Not lowering rents and leaving spaces vacant ‘maintains’ the value of the building, as long as you can keeep making the loan payments everyone pretends the building is worth more money than it probably actually is. As long as the borrower keeps making payments to the lender, nobody really cares.