I think to be fair we have been doing this in Europe for quite a while, many apartments were once factories or something else 100 years ago.

I think, from my understanding, the greatest challenge is in turning modern office blocks into housing. They are usually really big (10,000 -40,000 sqft) floor plates so there's very little natural light to go around and the shape of the flats needed to get window access would be really impractical. Meanwhile the slab to slab heights, floor loadings and locations mean they're not good for industrial or any other use beyond offices.