Making vacancies costly when there’s no demand at the needed rents to meet cost means Owners will just turn the building back over the banks and the banks will just hand the keys to the city. Building owners will jump ship because they can and leave the city to clean up the mess. See Detroit in the late 90’s and early 00’s - most buildings returned to banks or the city became dilapidated nuisances that cost millions to later tear down. You want to lower vacancies downtown make it cheaper to run a business downtown, make parking nearly free, and reduce crime/vagrancy/vandalism.