Is that a old style cast iron radiator against the far wall under the window?? 

Some investments would probably go down - most of the ones we typically think of as investments, most likely (real estate, stock, etc) - and other items (let's call them investments) would become super-valued - radiation-free food & water; geiger counters; weaponry for fighting off zombie hordes. 

The house used to be heated with radiators in all units.  

The radiator has been taking out, therefore the property need a new heating system.

I currently pay for the utilities but, without fail, I'll drive by the house during the winter and see windows open because the tenants are too ____________ to bend down and adjust the individual radiators in their apartments.

Lastly, I do see an old-timey radiator unit in several of the photos. 

An alternative proposed by a recent HVAC company was $1100 to flush the system, bleed the radiators, and leave the existing boiler in place, but install an ecobee wifi thermostat where I can set the max allowable temperature remotely.

Or maybe a baseboard electric radiator.

Originally the apartments were heated with hot water baseboard radiators and tenants used window AC units.

Removed boiler, radiators, and replaced with new Forced and Central Air.