It's not prohibitively expensive... office to residential conversions have already been done in many cases.

Of course the specifics of the building matter a lot, but it's certainly cheaper than ground up net new development.

And even if the conversion leads to abnormal units per modern standards, it will still get occupied at the right price.

Seems a no brainer to rezone most office space to residential. Free market will figure it out. You can bet buying these commercial properties at firesale prices will open up a profitable path to residential