Yeah not just the HVAC but the entire common area maintenance and pro rata property tax is a high portion of commercial rent in malls. A commercial tenant for the same space would probably pay 5x to 10x what an equivalent residential market rate would be.
Conversions are really expensive. If the property won't be sustainable with residential rent then why bother doing the conversion in the first place? Seems like a frustrating way to burn money.
I'm not saying it's impossible, but people drastically underestimate the complexity and cost of converting a commercial property into a residential one. All the codes are different, and all the requirements are different.
Plus these old malls are often garbage buildings which is why they aren't viable as a mall anymore anyway. Better to send a few million cleaning and preparing the site and then build something that's actually viable for housing.
At the end of the day, the buildings in a deadish mall or shopping center aren't worth much. Maybe the land is worth something but, in that case, why not start fresh with a purpose-built development, either housing or a mix of housing and retail.