You're leaving out the usage/ capacity side. If you actually fill all the empty spaces, you'll need to add additional facilities. Current facility usage is usually near capacity already.
Sure, but the real question is what does one consider productive enough? Should we tax a single family home at the rate of an apartment building just because the zoning changed? My opinion is that this type of scheme will be abused to force some people out of their homes in the name of benefiting society. My guess is developers and apartment companies can "persuade" politicians in many areas to change zoning to force people out, possibly the "wrong" kinds of people.
If you want productive uses, then you'd be better off charging a vacancy tax to already developed property. You wouldn't be able to fill all those spaces because the physical economy has shrunk, or people don't want to live/work in a rundown area, etc. There's plenty of open commercial real estate around me. The problem is you don't have customers to support the businesses that already exist.