Imagine apartments built into what used to be department stores, (Oh, you’re JC Penny 203? I’m at Sears 106). Get those old arcades up and running. Set up meal stations at the food court. Once people actually live there, stores will start to move back in.
If I’m unable to finish my life in my own home, that doesn’t sound like a terrible option.
Malls are usually multiple stories. Couldn’t you do all the plumbing on the first floor to avoid going underground. And then add structural reinforcement at the first floor to prevent higher floors from collapsing / sinking? Yeah for egress you’d have to put the bedrooms along the exterior walls. But I’m sure you could still get plenty of use over the space which does not border an exterior wall both as part of the unit and as part of some shared community area.
Watch when an old mall is demolished. Look at the structure and the way the heavy equipment is used. You’ll start to see the issue better. It is usually sketchy business to demolish because of the spans involved. It is more like a light bridge construction than it is a typical building. It can’t be demolished incrementally like taking out bites of the structure and working across. The ones I’ve seen drop the roof first and clean up what remains.
Only an engineer can really say one way or another, and every building will be different. However, I’m willing to bet the load bearing structure is specialized and unique to the application. I doubt there is any flexibility for repurposing more than 10-15% outside of the tolerance you see utilized. It is like the question why can’t a Honda Civic perform like a Bugatti. Technically anything is possible if you point your money machine gun at it in a currency downpour, but is it still a Civic when a Bugatti is cheaper?
The smarter option is likely to demolish the light structure of the department store and build a proper apartment building that is then attached back onto the mall corridor.