That's not how it works. You don't get to write off foregone rent as a loss, only the actual cost of running/maintaining the building.
The guy you replied to had it right; accepting a cheaper rent would cause the building's value to plummet (buildings are valued as a multiple of rents), which would plunge the owners into bankruptcy as they would now owe more than the building is worth. But you can survive a temporary market downturn by never accepting a lower rent so long as it comes back up in time.
The whole thing is silly, and the market is rife for a saner way to valuate properties. Personally I would use a decay factor on top of the last paid rent for vacant properties. E.g. the imputed rent for a vacant lease would decrease by 2% every month (this figure is open to fine-tuning), so that the property is worth less and less with every month that it cannot find a new tenant at its previous rent. Then, when a new actual rent is established and is being paid, you use that figure.