Well, sure, a wall could be painted and new guests in at under 4 hours, but really, you'd have to have a staff person all ready to go, paint match on hand(that would still match porperly and not make one wall look a different color from others walls having faded, cleaned too often, etc.), the furniture would need to be moved, floor coverings put down, maybe windows taped, etc., and then still have time for the regular room cleaning to be done. Sure, it's possible, but logistically, especially if on any given day even a small percentage of rooms needed extra attention of some sort, that could still be an issue getting the next guest checked in in a timely fashion. Yes, Magic Eraser type of product would be great...I don't know if they use that..and I agree...housekeeping really SHOULD do a better job and rooms should be inspected much better before and after each guest, but having a merry crew able to zip to any room with a quick fix in a short period of time can be much more difficult than it sounds
I believe that Disney is truly wanting to keep the guests happy, within parameters of what can be done, but we can't know their priorities or how other things can impact what they would ideally like to do. If a union is involved that can also have an effect, and in any huge company there are odd ball rules in place almost impossible to get over, and managers have to often their pick their fights. Until we know the behind the scenes workings, we can't say "well, it would be simple to just do this..." sure it would, but things aren't always simple through a thousand layers of big business.