I've seen proposals for this sort of thing in the past (supposedly UPS and/or FEDEX were looking at this to use their freighters during their daytime down hours).
But freighter aircraft are not equipped for passenger evac in an emergency. That's a show stopper for commercial (military may have different requirements). Also having spent dozens of hours on the main deck of a 747 Freighter during flight testing, it is NOT a pleasant place to be
. Takeoff and landing, with no windows or outside reference was particularly nasty
Something like this would bring 'cattle car' accommodations to a new low.